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Applying Rational Equations 1 Extra Commentary

Video vocabulary

for

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US / fə(r) /

UK / fɔr,fə /

A1
  • preposition
  • Used to show the purpose, or need of something

time

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US / taɪm /

UK / taɪm /

A1
  • noun
  • Speed at which music is played; tempo
  • Point as shown on a clock, e.g. 3 p.m
  • Number of hours, minutes needed to do something
  • Occasion when something happens
  • Period or occasion that something occurred
  • Period in history or the past
  • Something measured in minutes, hours, days, etc.
  • How long an event takes; duration
  • verb
  • To check speed at which music is performed
  • To choose a specific moment to do something
  • To measure how long an event takes, e.g. a race
  • To schedule something to occur at a specific moment

why

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US / waɪ /

UK / hwaɪ, waɪ /

A1
  • adverb
  • Question word to ask for a reason
  • exclamation
  • (Used to express surprise or approval)

spend

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US / spend /

UK / spɛnd /

A2
  • verb
  • To use money to pay for something
  • To use time in order to do something
  • To use up something completely

algebraic

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US / ˌældʒɪ'breɪɪk /

UK / ˌældʒəˈbreɪk /

C1
  • adjecitve
  • Concerning algebra

hour

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US / 'aʊə(r) /

UK / aʊr /

A1
  • noun
  • 60 minutes
  • Particular time of the day
  • Fixed, regularly occurring time

those

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US / ðəʊz /

UK / ðoz /

A1
  • determiner
  • The people or things we are talking about
  • pronoun
  • The objects far from the speaker

just

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US / dʒʌst /

UK / dʒʌst /

A1
  • adjecitve
  • Proper or appropriate; as is deserved
  • Doing or being what is right or fair
  • Legally correct; having enough evidence
  • adverb
  • Almost; nearly
  • Exactly at the time of
  • Exactly (the same as, like)

few

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US / fju: /

UK / fju /

A1
  • pronoun
  • Not many, a small number

much

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US / mʌtʃ /

UK / mʌtʃ /

A1
  • adjecitve
  • Very; to a high level or degree
  • adverb
  • A lot; large amount; a high degree of
  • Nearly; basically
  • On many occasions; often
  • noun
  • A large amount; a great quantity

ask

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US / ɑ:sk /

UK / æsk /

A1
  • verb
  • To say to someone that you want something

express

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US / ɪk'spres /

UK / ɪkˈsprɛs /

A1
  • adjecitve
  • Sent by a fast mail service
  • Traveling at high speed
  • noun
  • Fast train or bus
  • verb
  • To send something by fast mail
  • To state or show what you think or how you feel

well

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US / wel /

UK / wɛl /

A1
  • adjecitve
  • Being in a fortunate, good condition or state
  • Healthy and in good condition
  • adverb
  • In an acceptable, proper, or successful manner
  • In a complete, total, or full manner
  • A great and considerable degree
  • In a successful way, in a good or satisfactory way
  • exclamation
  • Used to begin or continue a particular statement
  • Used to show you want to start saying something
  • Used to express that you are waiting for a reply
  • You say this to change the topic of a conversation
  • Used to indicate you are making a final remark
  • noun
  • Long deep hole in the ground to get water
  • verb
  • To cause your eyes to fill with tears

basis

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US / ˈbeɪsɪs /

UK / ˈbesɪs /

A2
  • noun
  • Main ingredient or part of something
  • Starting situation, fact, idea to develop from

arbitrarily

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US / 'ɑ:bɪtrəlɪ /

UK / ˌɑrbəˈtrɛrəlɪ /

B1
  • adverb
  • Without using reason, often unfairly; on a whim

question

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US / 'kwestʃən /

UK / ˈkwɛstʃən /

A1
  • noun
  • Issue or problems you are dealing with
  • What you ask about; issue
  • verb
  • To ask for or try to get information
  • To have or express concerns or uncertainty

wonder

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US / 'wʌndə(r) /

UK / ˈwʌndɚ /

A1
  • noun
  • Surprise caused by experiencing something amazing
  • verb
  • To feel curious about something
  • To think or consider about something over time

per

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US / pə(r) /

UK / pɚ /

A2
  • preposition
  • In the manner directed or suggested
  • For each; during each

voodoo

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US / ˈvu:du: /

UK / ˈvudu /

A1
  • noun
  • Cult practiced by sorcery and spirit possession

should

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US / ʃʊd /

UK / ʃʊd /

A1
  • other
  • Used to indicate what is proper or reasonable

eat

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US / i:t /

UK / it /

A1
  • verb
  • To put food in your mouth

combine

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US / kəmˈbaɪn /

UK / kəmˈbaɪn /

A2
  • verb
  • To mix several things together to form one thing

sure

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US / ʃʊə(r) /

UK / ʃʊr /

A1
  • adjecitve
  • Having no doubts about something
  • Have confidence in your own words or actions
  • Firmly established or held; being firm or stable
  • Being correct or true
  • adverb
  • What you say to agree willingly

either

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US / ˈaɪðə(r) /

UK / ˈiðɚ, ˈaɪðɚ /

A1
  • adjecitve
  • One of two (used when there is a choice of two)
  • adverb
  • (After negative statements) likewise, in addition
  • conjunction
  • Used as the first of two connected things
  • determiner
  • Used as the first of two connected things
  • pronoun
  • The one or the other

intuition

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US / ˌɪntjuˈɪʃn /

UK / ˌɪntuˈɪʃən, -tju- /

B2
  • noun
  • Natural ability to guess or feel things

here

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US / hɪə(r) /

UK / hɪr /

A1
  • adverb
  • In this place; at this location, position or time

quick

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US / kwɪk /

UK / kwɪk /

A1
  • adjecitve
  • Using very little time; moving far in little time
  • Learning and understanding rapidly and easily

realize

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US / 'ri:əlaɪz /

UK / ˈriəˌlaɪz /

A1
  • verb
  • To become aware of or understand mentally

speak

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US / spi:k /

UK / spik /

A1
  • verb
  • To be able to use a certain language
  • To use words to tell information, express thoughts

write

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US / raɪt /

UK / raɪt /

A1
  • verb
  • To compose letters and words on paper or a screen
  • To invent or create a computer program
  • To create a song or piece of music

guess

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US / ɡes /

UK / ɡɛs /

A1
  • noun
  • Answer given when you don't know if it is correct
  • verb
  • To give an answer without knowing if it is correct

intuitive

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US / ɪnˈtju:ɪtɪv /

UK / ɪnˈtuɪtɪv, -ˈtju- /

B2
  • adjecitve
  • Able to understand by feeling rather than fact

many

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US / 'menɪ /

UK / ˈmɛni /

A1
  • determiner
  • Used to refer a large number of things

will

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US / wɪl /

UK / wɪl /

A1
  • noun
  • Desire or choice of someone
  • Document saying who gets your money when you die
  • Strong desire to do something; determination
  • Desire to do something; strong wish to do
  • other
  • Used with verbs to express the future
  • verb
  • To leave property to someone after your death
  • To use mental effort to make something happen
  • Am (is, are) likely to do
  • To influence someone to do something
  • To make something happen by strongly wanting it

plus

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US / plʌs /

UK / plʌs /

A2
  • noun
  • A benefit or advantage
  • preposition
  • In addition; add
  • In addition to; Indicates a number above
  • Having a value greater than zero

same

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US / seɪm /

UK / sem /

A1
  • adjecitve
  • Being equal in quality and amount
  • Identical; not different
  • Not changing; constant

good

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US / ɡʊd /

UK / ɡʊd /

A1
  • adjecitve
  • Proper, appropriate or right
  • (Of an amount) enough; plenty
  • Excellent; high quality
  • Acting well or properly; of moral character
  • In a positive or happy emotional state
  • noun
  • Advantage or benefit

finish

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US / ˈfɪnɪʃ /

UK / ˈfɪnɪʃ /

A1
  • noun
  • End of something; completion
  • Final coat (of paint) applied to a surface
  • verb
  • To use or consume all of something
  • To reach the end; to complete something
  • To put a final surface (e.g. paint) on something

but

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US / bət /

UK / bʌt,bət /

A1
  • conjunction
  • Used before you say something different, opposite

can

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US / kæn /

UK / kən,kæn /

A1
  • noun
  • A metal container for storing food
  • other
  • Able to; have the skill to; have the time to
  • verb
  • To have permission to
  • To be possible to do
  • To preserve food by sealing it in a metal container

equal

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US / 'i:kwəl /

UK / ˈikwəl /

A1
  • adjecitve
  • Same in shape, size, or number
  • verb
  • To add up or be the same as

now

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US / naʊ /

UK / naʊ /

A1
  • adverb
  • At the present time or moment
  • Right away or in the next possible moment
  • Used to refer to something that just happened
  • Time between the present and a moment in the past
  • conjunction
  • Resulting from or because of something

not

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US / nɒt /

UK / nɑt /

A1
  • adverb
  • Word indicating the negative
  • (Used to form the negative of verbs)

our

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US / 'aʊə(r) /

UK / aʊr /

A1
  • determiner
  • Belonging to us

really

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US / ˈri:əli /

UK / ˈriəˌli, ˈrili /

A1
  • adverb
  • Used to stress something that is said; definitely
  • In a true or accurate manner
  • Very

pick

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US / pɪk /

UK / pɪk /

A1
  • noun
  • Sharp tool used for breaking the ground
  • Item selected from a group of choices
  • verb
  • To take fruit or vegetables from a plant or tree
  • To decide on a thing from various choices; select
  • To remove (flower, etc.) with your fingers

would

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US / wʊd /

UK / wʊd /

A1
  • other
  • Used to indicate something imagined
  • Used to ask or request something politely
  • Verb used to indicate possibility, request etc.
  • Used to indicate a preference
  • Used to indicate a reaction
  • Indicates something that occurred repeatedly
  • Used to indicate willingness or ability to do

video

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US / 'vɪdɪəʊ /

UK / ˈvɪdiˌo /

A2
  • adjecitve
  • Of a moving film, with sound
  • noun
  • Recording capturing action with sound
  • verb
  • To make a moving film of, with sound

want

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US / wɔnt /

UK / wɑnt, wɔnt /

A1
  • noun
  • Something you desire or wish for
  • State or condition of poverty
  • verb
  • To desire or wish for something; hope for a thing

sense

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US / sens /

UK / sɛns /

A1
  • noun
  • Certain mental feeling or emotion
  • Normal or clear state of mind
  • Meaning of a particular word, phrase or text
  • verb
  • To perceive using sight, sound, taste touch etc.
  • To recognize the presence of something

problem

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US / ˈprɒbləm /

UK / ˈprɑbləm /

A1
  • adjecitve
  • Causing trouble
  • noun
  • Something difficult to deal with or causes trouble
  • Question to show understanding of a math concept

ratio

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US / ˈreɪʃiəʊ /

UK / ˈreɪʃioʊ /

C2
  • noun
  • The relationship in relative size, etc.

behind

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US / bɪˈhaɪnd /

UK / bɪˈhaɪnd /

A1
  • adjecitve
  • Concerning a later or slower time than expected
  • Being in a losing position in a sporting event
  • Being in the past; old fashioned
  • adverb
  • Remaining
  • noun
  • The part of the body that one sits on
  • preposition
  • At or toward the back of someone or something
  • Supporting or helping someone

could

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US / kəd /

UK / kʊd /

A1
  • other
  • Past form of 'can' to mean have ability
  • Used to ask for something politely, or to offer

have

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US / həv /

UK / hæv /

A1
  • verb
  • To drink, smoke, eat or use something
  • To experience the effects of something
  • To organize an event, or join in an activity
  • To own, possess, or hold something
  • To cause to happen or produce a particular effect

unit

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US / ˈju:nɪt /

UK / ˈjunɪt /

B2
  • noun
  • One apartment in a building
  • Group of people, e.g. soldiers, that work together
  • A chapter of a textbook
  • A standard of measurement of something
  • Single item made by a company

independently

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US / ˌɪndɪ'pendəntlɪ /

UK / ˌɪndɪˈpɛndəntlɪ /

A2
  • adverb
  • In a manner being free from the control of another

between

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US / bɪˈtwi:n /

UK / bɪˈtwin /

A1
  • preposition
  • From one person, thing, or place, to another
  • In the space that separates two objects
  • Among two or more people who share something

they

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US / ðeɪ /

UK / ðe /

A1
  • pronoun
  • Two or more people, animals, or things

add

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US / æd /

UK / æd /

A1
  • verb
  • To include as well
  • To put two things or numbers together, e.g. 2+2

take

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US / teɪk /

UK / tek /

A1
  • noun
  • Filming of a single scene for TV or the movies
  • verb
  • To accept something that is offered
  • To use a method of traveling, e.g. a bus or train
  • To use a camera to make (a photo)
  • To pick up something and go away with it
  • To study a subject in school
  • To attempt to pass (an exam)
  • To require a certain amount of time, money, space

both

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US / bəuθ /

UK / boθ /

A1
  • determiner
  • Used to refer to two things at the same time
  • pronoun
  • Used to refer to two things at the same time

their

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US / ðɛː /

UK / ðer /

A1
  • determiner
  • Belonging to them

way

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US / weɪ /

UK / we /

A1
  • adverb
  • Used to emphasize the amount of something
  • noun
  • Particular aspect of something being thought about
  • One of various choices that could be made
  • Particular existing condition or state
  • (Often long) distance or length of time
  • How something is done
  • Route to go to a place
  • Small street
  • Usual behavior, emotions or habits of someone

work

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US / wɜ:k /

UK / wɚk /

A1
  • adjecitve
  • Activity completed in order to make money
  • noun
  • The product of some artistic or literary endeavor
  • Everything created by an author, artist, musician
  • Activity you do in order to make money
  • Place of employment
  • Final product or result of a some labor or effort
  • verb
  • To bring into a specific state of success
  • To be functioning properly, e.g. a car
  • To do your job in your company or workplace
  • To make an effort to do something
  • To operate a machine or device

bit

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US / bɪt /

UK / bɪt /

A1
  • noun
  • Device put in a horse's mouth to control it
  • Small piece of something
  • verb
  • (E.g. of fish) to take bait and be caught

confusing

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US / kənˈfju:zɪŋ /

UK / kənˈfjuzɪŋ /

A2
  • adjecitve
  • Being hard to understand
  • verb
  • To make something unclear or hard to understand

all

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US / ɔ:l /

UK / ɔl /

A1
  • adverb
  • Completely; totally
  • determiner
  • Being every one of something
  • pronoun
  • 100% of something; the complete amount

fast

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US / fɑ:st /

UK / fæst /

A1
  • adjecitve
  • In a way that is difficult to move or change
  • Moving or able to move quickly
  • Showing time that is later than the actual time
  • adverb
  • With a great deal of speed
  • noun
  • Period of time when a person chooses not to eat
  • verb
  • To refuse food for a long time

only

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US / ˈəunli /

UK / ˈonli /

A1
  • adverb
  • Just one; just that amount or thing; not more than
  • Not very important or serious
  • Just that type of thing, person
  • Short time ago; not before a certain time
  • conjunction
  • Except; but

two

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US / tu: /

UK / tu /

A1
  • number
  • 2

and

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US / ənd /

UK / ənd, ən,ænd /

A1
  • conjunction
  • Used to refer to two or more things
  • Plus; in addition; on top of that
  • Used to introduce an action that follows another

incorrect

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US / ˌɪnkəˈrekt /

UK / ˌɪnkəˈrɛkt /

B1
  • adjecitve
  • Being wrong or inaccurate

hopefully

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US / ˈhəʊpfəli /

UK / ˈhopfəli /

A2
  • adverb
  • In a manner wishing something to happen

another

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US / əˈnʌðə(r) /

UK / əˈnʌðɚ /

A1
  • adjecitve
  • One more, but not this
  • determiner
  • One more added
  • pronoun
  • One more (thing)

hey

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US / heɪ /

UK / he /

A1
  • exclamation
  • What you say when you want someone's attention

dean

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US / di:n /

UK / din /

A1
  • noun
  • Official in charge of one part of a university

where

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US / hwɛə /

UK / hwɛr, wɛr /

A1
  • conjunction
  • At or in the place which
  • In whatever place
  • pronoun
  • Word you use to ask the location of something

decide

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US / dɪ'saɪd /

UK / dɪˈsaɪd /

A2
  • verb
  • To make a choice or choose after thinking about it
  • To make a legal judgment in court
  • to affect the result of something

effort

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US / ˈefət /

UK / ˈɛfət /

A2
  • noun
  • Amount of work used trying to do something

literally

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US / ˈlɪtərəli /

UK / ˈlɪtərəli /

B1
  • adverb
  • So as to express the exact meaning of something
  • Used to say something is true, even if unlikely

think

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US / θɪŋk /

UK / θɪŋk /

A1
  • verb
  • To have an idea about something without certainty
  • To have an idea, opinion or belief about something

about

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US / ə'baʊt /

UK / əˈbaʊt /

A1
  • adverb
  • To be ready to; going to
  • Nearly; approximately; roughly
  • Near; next to; close to
  • In every direction; in the area of
  • preposition
  • Concerning or of (a topic)

like

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US / laɪk /

UK / laɪk /

A1
  • adjecitve
  • Being equal in status or kind to something else
  • Similar; the same
  • adverb
  • For example
  • preposition
  • Used as a filler; used to ad emphasis
  • In a way similar to the way something is done
  • Such as; for example
  • verb
  • To want to
  • To find something pleasing; to prefer something

that

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US / ðæt /

UK / ðæt, ðət /

A1
  • adjecitve
  • The person or thing being talked about
  • Used to identify something both the speakers know
  • adverb
  • Used to reinforce adjectives and adverbs
  • To such a degree
  • conjunction
  • Used to connect a noun clause
  • Used to introduce an adverbial clause
  • Used to introduce a noun clause
  • determiner
  • Used as a noun to refer to something
  • pronoun
  • Referring an object far away
  • Used to refer to the relative pronoun ‘which’

what

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US / wɒt /

UK / hwɑt, hwʌt, wɔt, wʌt,hwət, wət /

A1
  • pronoun
  • Thing or things that; the same kind as
  • Used to emphasize something you are about to say
  • adjecitve
  • Used to refer to a particular amount, or number
  • Used to place emphasis on a particular quality
  • adverb
  • To a certain degree
  • determiner
  • Question words used for asking for information
  • exclamation
  • Used to express excitement, shock, or surprise
  • Used to ask people to repeat something they said

each

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US / i:tʃ /

UK / itʃ /

A1
  • determiner
  • Every one of two or more things
  • pronoun
  • Per one person or thing

happen

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US / 'hæpən /

UK / ˈhæpən /

A1
  • verb
  • To take place or occur

language

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US / ˈlæŋgwɪdʒ /

UK / ˈlæŋɡwɪdʒ /

A1
  • noun
  • Words or signs used to communicate messages

then

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US / ðen /

UK / ðɛn /

A1
  • adverb
  • A time that is not now
  • Following another thing in time or arrangement
  • Following on reasonably from what was just stated
  • At that time, not now

total

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US / ˈtəʊtl /

UK / ˈtoʊtl /

A1
  • adjecitve
  • Complete
  • noun
  • Whole number or amount of something
  • verb
  • To damage a car beyond repair

type

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US / taɪp /

UK / taɪp /

A1
  • noun
  • Printed letters or blocks used for printing them
  • Group of things or people sharing common features
  • verb
  • To identify something belonging to a group
  • To write by touching keys on a keyboard

number

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US / ˈnʌmbə(r) /

UK / ˈnʌmbɚ /

A1
  • noun
  • Symbols such as 1, 2, 56, 793
  • Particular song or dance performed during a show
  • Total quantity or amount of things
  • verb
  • To put numbers on things
  • To assign a sequence within a group, series, set
  • To claim to be part of a larger group, series, set

tell

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US / tel /

UK / tɛl /

A1
  • verb
  • To (strongly) advise a person to do something
  • To be able to guess
  • To say or communicate information to someone
  • To inform, but without speaking; indicate
  • To have a negative effect on someone
  • To order someone to do something; command
  • To recognize something based on certain signs
  • To report something to an authority
  • To explain something or teach someone something

get

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US / ɡet /

UK / ɡɛt /

A1
  • verb
  • To become affected by illness or disease
  • To begin to feel or understand an emotion or idea
  • To send or bring someone or something home, etc.
  • To go somewhere to obtain something
  • To (cause to) do a particular thing
  • To obtain, receive or be given something
  • To currently have
  • To prepare for use
  • To understand something being said or read

answer

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US / 'ɑ:nsə(r) /

UK / ˈænsɚ /

A1
  • noun
  • Reply to a question someone asks
  • Solution to a problem or test question
  • verb
  • To reply to a question someone asks
  • To solve a test question or a problem

less

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US / les /

UK / lɛs /

A1
  • adjecitve
  • Smaller in amount or degree
  • adverb
  • To a smaller degree
  • noun
  • A smaller amount
  • preposition
  • With the subtraction of

make

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US / meɪk /

UK / mek /

A1
  • noun
  • Product made by a particular company; brand
  • verb
  • To arrange or prepare something e.g. dinner
  • To get to (a place) on time
  • To gain the status of (teacher, lawyer, etc.)
  • To create something by putting things together
  • To earn a certain amount of money at a job
  • To cause or force a person to do something
  • To cause something to happen or be formed

one

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US / wʌn /

UK / wʌn /

A1
  • pronoun
  • You; we; people in general; someone
  • The thing or person previously referred to
  • adjecitve
  • Used to refer to people generally
  • Being a strong example of (something mentioned)
  • number
  • Number 1

other

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US / 'ʌðə(r) /

UK / ˈʌðər /

A1
  • adjecitve
  • A thing different from that mentioned
  • determiner
  • (Something) else; not the first (one)
  • pronoun
  • Being the one and only thing of that category

than

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US / ðən /

UK / ðən /

A1
  • conjunction
  • Used to compare two things that are not equal

how

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US / haʊ /

UK / haʊ /

A1
  • adjecitve
  • To what degree? in what amount?
  • adverb
  • (Used to explain the way to do something)
  • exclamation
  • For what purpose or reason?
  • (Used to ask someone or something's condition)

sal

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US / sæl /

UK / sæl /

A1
  • noun
  • A North Indian tree that yields hard, durable timber and dammar resin.

you

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US / jʊ /

UK / ju /

A1
  • pronoun
  • Person someone is speaking or writing to
  • Person or people in general

flip

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US / flɪp /

UK / flɪp /

B2
  • noun
  • Act of turning your body in the air; somersault
  • Movement of something from one position to another
  • verb
  • To turn your body in the air, as in gymnastics
  • To move into a different position quickly
  • To move a button or switch to turn on or off

this

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US / ðɪs /

UK / ðɪs /

A1
  • adjecitve
  • Used to indicate something already been discussed
  • Used to indicate something happening around now
  • adverb
  • An indication by a physical motion or gesture
  • A specific amount of
  • determiner
  • Person, thing, or idea near you
  • pronoun
  • Person, thing that has already been discussed

term

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US / tɜ:m /

UK / tɚm /

A1
  • noun
  • Conditions applying to an agreement, contract
  • Length of time something is expected to happen
  • Fixed period of weeks for learning at school
  • The (precise) name given to something
  • verb
  • To call; give a name to

half

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US / hɑ:f /

UK / hæf, hɑf /

A1
  • adjecitve
  • Not complete
  • adverb
  • Partly; somewhat
  • noun
  • One of two equal periods of game time in a sport
  • The part you get when one is divided into two;

little

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US / 'lɪtl /

UK / ˈlɪtl /

A1
  • adjecitve
  • Being short in length of time
  • Not great in size, amount, or degree; small
  • Being very young or younger than others
  • adverb
  • By an amount or degree that is not very much
  • Not often; rarely
  • noun
  • A person's name

let

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US / let /

UK / lɛt /

A1
  • verb
  • To allow someone to do something
  • Introducing a suggestion to do something together
  • To rent a house, etc. to others

might

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US / maɪt /

UK / maɪt /

A1
  • other
  • Used to politely request an action
  • Used to express that something could happen
  • Power, strength, force or influence

rate

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US / reɪt /

UK / ret /

A1
  • noun
  • Current price or cost of something
  • Speed or frequency of events over time
  • verb
  • To assess something or consider its qualities

the

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US / ðə /

UK / ðə /

A1
  • article
  • Used to refer to something already mentioned
  • Used to show there is only one of something
  • determiner
  • Used to describe all of a family
  • Subtitles section
  • I want to make a quick note in this video because I realized that some of these ratio and rate algebraic problems can get a little bit confusing.
  • So I just want to make sure you get the intuition behind it.
  • So if I were to tell you that Sal takes-- I don't know-- let's say he takes 1 hour to eat a pie.
  • Let me pick a number better than 1.
  • Let's say he takes 2 hours.
  • 2 hours to eat a pie.
  • So he takes 2 hours per pie.
  • And let's say Dean takes 3 hours to eat a pie.
  • So he takes 3 hours per pie.
  • And if I were to ask you, how long does it take for both of them, if they're both working together, how long will it take them to eat a pie?
  • You might be wondering, well, why can't I just add the 2 hours per pie and the 3 hours per pie?
  • Why can't I just do 2 hours per pie plus 3 hours per pie?
  • Why would this be incorrect?
  • Why wouldn't this be their combined rate?
  • I guess the best way to think about it is think about what's happening here.
  • You're saying that Sal takes 2 hours a pie, Dean takes 3 hours a pie.
  • If they're both working on the same pie, it's going to take them less time.
  • They're going to do it faster.
  • But when you add the hours, what are you doing?
  • You're going to get 5 hours per pie.
  • This is not answering the question, how many hours will it take for them to finish one pie?
  • This is answering the question, how much total time would be spent if they each independently ate a pie?
  • Sal would spend 2 and then Dean would spend 3.
  • Their total time would be 5.
  • So we're just adding times here.
  • If you really want to combine their effort, you want to combine their rates, you have to express both of these in terms of rates.
  • And if you want to express 2 hours per pie in terms of rate, you would say that this is the same thing as-- you would just write the ratio the other way around.
  • Instead of 2 hours per pie you would say, 1 pie per 2 hours, for 2 hours.
  • Or you could view it as 1/2 pie per hours.
  • Now it is expressed as a rate.
  • Do the same thing for Dean.
  • Instead of 3 hours per pie, you could say that he would do 1 pie every 3 hours.
  • Or another way to think about it is he does 1/3 of a pie per hour.
  • Now we're speaking in a language of rates.
  • How many pies per hour I can do, how many Dean can do.
  • If we combine it, then we have our combined rate.
  • So if we add 1/2 plus 1/3, that is our combined rate in pies per hour.
  • And what is this equal to?
  • 1/2 is 3/6.
  • Plus 1/3 is 2/6, which is equal to 5/6.
  • All the units here are in pies per hour.
  • And then if we wanted to say, how many hours?
  • So we would do literally 5/6 of a pie per hour when we're eating together.
  • Now if you want to say how many hours per pie, you would just write this ratio.
  • You would just flip it.
  • So this is the same thing as 6/5 hours per pie.
  • So hopefully that makes sense.
  • I don't want this to seem like some type of voodoo where I just decide arbitrarily to flip both of these numbers.
  • I'm flipping them so I can express them as rates.
  • Which hopefully makes intuitive sense because you can only add rates.
  • If you add the times, you're not saying that they're working on the same pie.
  • You're saying how much total time is it taking them.
  • Only when you're flipping them and combining you're saying, hey, their rates are being combined on the same effort.
  • I can combine them.
  • And that'll tell you their combined rate.
  • And then you can take the flip of that and say, on a combined basis, how many hours?
  • And it should make intuitive sense.
  • If we're both eating the same pie, it's going to take fewer hours than either of us.
  • And 6/5 is 1 and 1/5 hours or 1.2 hours.
  • Or 1 hour and 12 minutes, which makes sense.
  • It's a little bit more than if you had two Sals.
  • Two Sals would take 1 hour.
  • And two Deans would take an hour and a half.
  • So it's someplace in between those two numbers.
  • Which hopefully, make sense.