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## Dividing fractions by fractions

# Dividing mixed numbers

CCSS.Math:

## Video transcript

- [Tutor] Let's see if we can figure out what four and four fifths
divided by one and one half is and I encourage you to pause the video and see if you can
figure it out on your own and I'll give you a hint, see if you can rewrite these mixed numbers as what is sometimes
called improper fractions. Alright, now let's do this together, so how can we rewrite
four and four fifths? Well, four and four
fifths is the same thing as, if we take the four,
that's the same thing as four plus four fifths,
four plus four fifths and four is the same thing as 20 fifths, so 4 is the same thing as 20 fifths and then plus four fifths,
well, what are you going to get? Your going to get, you add the numerators, you get 24 fifths, 24 fifths, another way to think about
it is take this denominator, take the fifths, multiply
by four, you get 20 fifths, plus the four fifths that
you already have is 24 fifths and so this is the same
things as 24 fifths divided by, use the same idea, one and a half is the same
thing as one plus one half, one is the same thing as two halves plus one half and so that's
going to be, add the numerators, that's going to be three
halves, so just like this, we're able to rewrite our expression as 24 fifths divided by three halves and now the key realization is is that that is the same thing as 24 fifths times the reciprocal of three halves, so times, pause the video, what's the reciprocal of three halves? Well, the reciprocal of three halves, you just swap the numerator
and the denominator, it's going to be two over three, now what is this going to be? Well, there's a couple of ways to do it, you could just straight
up multiply the numerators and you would get 48 and then
multiply the denominators and you would get 15,
so you get 48 over 15, but you might be able to rewrite that in sometimes what's called
a more simplified way, but another way of thinking about this is you could just say,
well this is the same thing as 24 times two, times two over five times three and simplify things, before you even multiply them out, five times five times three and you realize that look, 24 and three are both divisible by three, so let me divide them both by three, so 24 divided by three is eight and three divided by three is equal to one and then you could multiply the numerators and the denominators and so, and so you get in the numerator, eight times two is 16, in the denominator, you get a five, so you get 16 fifths and then if you wanna express
that as a mixed number, 16 over five, well, five goes into 16 three times with one left over, so this is three and one fifth and one thing to
appreciate, right over here, I simplified that the 24
and the three of this step, sometimes you'll see people
simplifying at this step, so they'll say, hey look,
eventually I'm gonna have a 24 in the numerator and
a three in the denominator, so let me divide both of those by three, so they'll say 24
divided by three is eight and then three divided by three is one and this is sometimes
called a cross reduction, but this is all that's
going on right over here.