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## 6th grade

### Unit 2: Lesson 4

Dividing fractions and whole numbers# Dividing a whole number by a fraction with reciprocal

Think about how many groups of a fraction are in 1, then scale up to find how many groups are in a larger whole number. Created by Sal Khan.

## Want to join the conversation?

- this confusing(12 votes)
- this
**is**confusing(10 votes)

- I just realized watching long videos and afking can be a energy farm(6 votes)
- if you pause evvery second it gives you even more energy LOL(3 votes)

- so are you just fliping the numarator and denominator is that what it basicaly is for reciprocal?(4 votes)
- 1:24But it isn’t (1/5) it’s (1/7). The whole, here a rectangle, is divided into 7 parts. 😫😩😭(2 votes)
- In the rectangle, the whole is 7/7. But that 7/7 is equal to 7/5. Thus, 1 is still 5 of the 7 on the rectangle, or 5/7 as Sal notes. I did not hear Sal talk about 1/5 or 1/7.

It also makes sense mathematically, Sal states that 5/7 of 7/5 is 1, of means times in math, so if you multiply 5/7 * 7/5, 7/7=5/5 and the whole thing ends up as 1.(2 votes)

- How many groups of 1/8 are in 3(1 vote)
- Since each whole has 8/8, 3x8=24. So there are 24 1/8 in 3.(2 votes)

- From all your related questions, it appears you need to work on dividing fractions and whole numbers.

There are multiple possibilities, but the process is the same for any of them.

Fraction in numerator, whole number in denominator

(2/3)/4 so divide 4 by 1 to get (2/3)/(4/1) and use keep change flip to get 2/3 * 1/4 = 1/6.

Whole number in numerator 2/(3/4) you could divide 2 by 1 at least in the beginning to make it easier (2/1)/(3/4) and use keep change flip to get 2/1 * 4/3 = 8/3.

Then there could be fractions in both numerator and denominaotr, (1/2)/(3/4) use keep change flip to get 1/2 * 4/3 = 2/3.

As noted in one of your other questions, you could then have mixed numbers in the numerator, denominator, or both which adds the step of turning the mixed number into an improper fraction, then use keep change flip.(5 votes)

- I watch the video but it is saying that I have not watched it and it is not putting the check by the video like the rest of them but I watch all three of them(1 vote)
- I know about that. Don't rewind the video and you will get a mark.(1 vote)

- This is so confusing and math is so hard but what is 1+1. Get it correct you gett a lolly pop :)(1 vote)
- I watch the video but don't get credit?(1 vote)

## Video transcript

- [Instructor] In this video,
we're gonna do an example that gives us a little bit of practice to think about what does it
mean to divide by a fraction? So if we wanna figure out what eight divided by 7/5 is, but we're gonna break
it down into two steps. First of all, we're gonna
use these visuals here to think about how many
groups of 7/5 are in one. Or another way of thinking about it is how these 7/5 are in a whole? So pause this video, and just
think about this first part. All right, so let's look at 7/5. 7/5 is everything from
here, all the way to there. And then one is this. So how many 7/5 are in one? Well, you can see that one, which is the same thing
as 5/5, is less than 7/5. So it's actually going
to be a fraction of a 7/5 that is one, or that is in one. And you can see what that fraction is. One is what fraction of 7/5. Well, if you look at the fifths, 7/5 is of course seven of
them, and a whole is 5/5, so five of the 7/5 make a whole. So the answer right over here is 5/7. 5/7 of a 7/5 is equal to one. You can also see this right over here. If you take each of these to be a fifth, each of these to be a fifth, then this whole bar is equal to 7/5. And the blue part is equal to one. So how many 7/5 are in the blue part? Well, we can see it's
5/7 of the whole bar. Once again, 5/7 of the whole bar. So we can also think about this as one divided by 7/5. This is another way of saying
how many 7/5 are in one, or how many groups of 7/5 are in one? And this is equal to 5/7, which we've learned in other videos is the reciprocal of 7/5. The numerator and the
denominator is swapped. So now what is eight
divided by 7/5 going to be? Well, if one divided by 7/5 is 5/7, or if you have 5/7 of a 7/5, of a 7/5 in one, I know the oral language
gets a little bit confusing. Well, you're going to have
eight times that many in eight. So this is going to be the same thing as eight times, we could do it this way, eight times one, divided by 7/5. Or you could just view this as eight times the reciprocal of 7/5,
which is five over seven. And we've learned how
to multiply this before. Eight times 5/7 is going to be equal to 40/7, and we're done. You could obviously also
write that as a mixed number if you like, this would be
the same thing as 5 5/7. So the big picture is when we think about how many of a fraction are in one, that's the same thing as saying, what's one divided by that fraction? As you see visually here, you essentially get the
reciprocal of that fraction. And so if you take any
other number other than one divided by that fraction, you're essentially just gonna multiply it by that reciprocal, because
it's that number times one. So when you divide by that fraction, it's that number times the reciprocal.