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## 6th grade (Illustrative Mathematics)

### Course: 6th grade (Illustrative Mathematics) > Unit 4

Lesson 2: Lesson 10: Dividing by unit and non-unit fractions# Dividing a whole number by a fraction with reciprocal

Dividing by a fraction involves finding the reciprocal and multiplying. For example, when dividing 8 by 7/5, first find the reciprocal of 7/5, which is 5/7. Then, multiply 8 by 5/7, resulting in 40/7 or 5 5/7. This method simplifies fraction division and enhances understanding. Created by Sal Khan.

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- A reciprocal is like a unique partner for a number. When you multiply a number by its reciprocal, you get 1. For example, the reciprocal of 5 is 1/5, and if you multiply 5 by 1/5, you get 1. The same goes for other numbers.

If you have a fraction, like 2/3, the reciprocal simply flip the fraction upside down, becoming 3/2. When you multiply 2/3 by 3/2, you get 1.

Remember that multiplying by a number and dividing by its reciprocal is the same. So if you want to divide by 4, you can multiply by 1/4 instead.

UPVOTE PLEASE(50 votes) - I just realized watching long videos and afking can be a energy farm(17 votes)
- Roses are red violets are blue who else is looking though the comment bc they bored to(15 votes)
- This is so frustrating.

I don’t get it at all.

When I think I got it and I try to do it, it ends up the wrong way!(4 votes)- Can you give a couple of examples that you worked and and got incorrect? Maybe this will help where you are going wrong.(5 votes)

- bro drew a perfect circle(4 votes)
- so are you just fliping the numarator and denominator is that what it basicaly is for reciprocal?(4 votes)
- so does it just mean that the group is just the reciprocal to get how many groups are there?(3 votes)
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## 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.