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## 5th grade foundations (Eureka Math/EngageNY)

### Course: 5th grade foundations (Eureka Math/EngageNY) > Unit 4

Lesson 7: Topic G: Foundations# Relating division to multiplication

Sal talks about the relationship between multiplication and division problems.

## Want to join the conversation?

- What does division have to do with multiplication?(16 votes)
- They are directly related to each other. Say you have 12 / 3 = 4. With this, you know that 3 x 4 = 12.(9 votes)

- Is this sort of the start of algebraic thinking?

( doing ( _ / 2 = 9 ) other than ( 9 * 2 = _ ) ? )(13 votes)- one of the cool new things I see in this algebra stuff (which coincidentally I'm relearning right now) is that in algebra we begin to tolerate unknowns, working WITH the unknowns to arrive at the solution we're looking for. at first you usually are solving for x but gradually you learn to solve for y or z and allow x to just be x....the system still works and the rules remain the same.

eventually we come to f(x) which has a shape, identity, behavior we know (like a fractal or a sine wave) without ever having to wring any specific number out of little x

if it's all in the same system you don't always have to answer EVERY question or every portion of every question, look for what you need. work with the system to derive it

especially if your testing/contest system is using multiple choice, eliminate the wrong answers and see what's left to pick

but I digress

yes! _/2=9 and instead of "blank" you can say/write x

this is the beginning of algebra

good catch Zhang(17 votes)

- Okay, just putting this out there, when I'm doing math, sometimes I get confused with all of the symbols: ×,+,-,÷ etc. And I end up dong the wrong thing, like subtacting instead of adding. Does anyone have a trick they use to remember what means what? Or is it just that I have to pay more attention to the symbols when doing problems? ;P(2 votes)
- Both of the ones that make the number go up are a X shape and the ones smaller have a -- in them(2 votes)

- no idea at all...(2 votes)
- So say you have a home work assignment for math class, and a question asks you, "Oh, jjcdcv, fill in the blank, 49 divided by blank equals 7" So obviously you know multiplication because that's one of the the first things they taught you, so you're like "Well 7 X 7 = 49" So there's your answer 49 divided by 7 = 7.(2 votes)

- How can I relate division to multiplication when I'm working on Division?(2 votes)
- Think of it like this: When you divide 8 by 2 you are basically saying "What would I have to multiply 2 by to get 8." So instead of writing 8 / 2 = X you could write 2 * X = 8. Does this help?(2 votes)

- Is this the start of algebra with blanks and question marks?(2 votes)
- Yes, blanks and question marks are the start of algebra.(2 votes)

- is it reverse? than multipalcation?(2 votes)
- It is like reverse multiplication. Multiplication is a faster way instead of doing repeated addition, and division is a faster way instead of doing repeated subtraction.(2 votes)

- Why would someone walk up on you on the street and tell you that?(2 votes)
- Can we use Mutiplecation to check our work from division(2 votes)
- Yes, if you answer is correct you can multiply it by what you divided by to get the starting number. :)(1 vote)

- I have known right now.(1 vote)

## Video transcript

- [Voiceover] Let's say
someone walks up to you on the street and says to you, 50 divided by five, 50 divided by five is equal to blank, is equal to blank. How can you think about this as a multiplication problem? Well, whatever blank
is, whatever blank is, if you multiply it by five, if you multiply by five, you should get 50. You should get 50. You should get 50. So one way of thinking
about it, you could say "what times five is 50?" Well, 10 times five is 50. 10 times five is 50, so 50 divided by five is going to be 10. Hopefully you see the relationship here. If 50 divided by five is 10,
then 10 times five is 50. 10 times five is 50. And you could do it the other way around. What is 50 divided by 10 going to be? 50 divided by 10, well that's
going to be equal to five. 50 divided by 10 is going
to be equal to five. How do I know that? Well, five times 10, five times 10, five times 10, is equal to 50. Is equal to 50. So let's keep thinking about this. If someone walked up to
you in the street again, and said blank, blank, divided by, divided by two, blank divided by two is equal to, is equal to nine. How would you figure out what blank is? Something divided by two is equal to nine. Well, one way to think about
it, and if we just follow here, if you said 50 divided by five is 10, you could say 10 times five is 50, so right over here we could say well, nine times two, nine times two, must be equal to blank. Must be equal to our blank. Well we know what nine times two is, that is 18, so this must be 18. 18 divided by two is nine. And that's really describing how 18, two, and nine relate to each other. Two nines is 18, or nine twos is 18, or if I were to divide 18 into two groups, each group would have nine. Or if I were to divide into groups of two, you would have nine groups. Any way you look at it, 18 divided by two is nine,
nine times two is 18. Let's do one more of these. So, someone walks up to you on the street, and tells you, and tells you that 12 divided by, divided by blank, 12 divided by blank is equal to three. Is equal to three. What is blank going to be? Well one way to think about this is three times blank is
going to be equal to 12. Three times blank, three times this unknown number, is going to be equal to 12. And three times what is equal to 12? Well three times four is equal to 12, so 12 divided by four is equal to three.