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### Unit 14: Lesson 15

Adding and subtracting fractions: word problems# Fraction word problem: pizza

Sal uses fraction models to figure out what fraction of a pizza is left. Created by Sal Khan.

## Video transcript

Brandon ate 5 slices
of apple-- of pie. I'm just assuming
it's apple pie. They didn't tell me that. Gabriela ate 3 slices. If there were
originally 9 slices, what fraction of
the pie was eaten? So let me see if I can
draw this thing out. So let me draw the pie. I will draw the pie
in a yellowish color. So let me try my best. So let's see how good
I am at drawing a pie. So I'll just draw it from
the top view as a circle. And there're 9 slices. I think it's a
reasonable assumption to say that they're
9 equal slices. So we have 9 equal
slices of pie. And I'll just make sure they're
initially 9 equal slices. What fraction of
the pie was eaten? So let's first divide
this into 9 sections. So one way I could do that, I
could divide it into 3 sections first, so it looks
like a peace symbol. It actually looks more
like the Mercedes emblem. So I'll draw it into
3 sections first. Then I'll do each of those into
3 sections, and I'll have 9. So let's see, I'll draw
like that and like that. Keep in mind, I'm trying to
make these as equal as possible. So bear with me if they
don't look 100% equal, but I'm trying. I am trying my best. So 9 equal slices-- so that
looks pretty respectable. So here's our pie that
initially had 9 equal slices. Now, they tell us that
Brandon ate 5 slices of pie. So Brandon eats-- he seems
like a hungry young man-- so he eats 1, 2, 3, 4, 5. You could say that he
ate 5/9 of the pie. But that's not it. That's not what they're asking. It's saying total, not just
how much did Brandon eat, but how much was eaten total
between Brandon and Gabriela. And they tell us
Gabriela ate 3 slices. So she ate 1-- sounds
like they didn't really eat dinner-- 1, 2, 3. So now let's answer
their question. What fraction of
the pie was eaten? Well, we know that there was a
total of 9 equal slices of pie. What fraction was eaten? Well, as we see, 1,
2, 3, 4, 5, 6, 7, 8. 8/9 of the pie was eaten. So let's actually type that in. And then we will do
it right over here. We'd say 8, and we use
that little slash symbol on your computer like this. 8 over 9 or 8 divided by 9
or 8/9 of the pie was eaten. Let's do a couple more of these. Ishaan ate 2 slices of pizza. Omar ate 3 slices. If there were
originally 8 slices, what fraction of the
pie is remaining? So this is interesting. Actually, let me
copy and paste this so that I can do it on my
little notepad right over here. So let's do it right over--
trying to find some space. There you go. Let me put that in here. So the same thing,
we have-- well, this is a pizza now, not a pie. And so pizza I
will draw in brown. It has 8, and we can assume
it's initially 8 equal slices. So it's actually a little bit
easier to draw 8 equal slices, since 8 is an even number. So let's see. That is my best attempt
at drawing a circle. And let's see, first I can
divide the pizza into 2 slices, then I can divide
it into 4 slices. And I'm going to try to make
them look as equal as possible. And now I'm going to--
with two more cuts, I should be able to get 8 so--
and one more just like this. So there you have it, a pizza
that has 8 equal slices. Now, they tell us Ishaan
ate 2 slices of pizza. So he eats 1, 2 slices of pizza. Omar ate 3 slices. Omar ate 3. So he eats 1, 2, 1,
2, 3 slices of pizza. Now, you might
immediately say, oh, OK, the answer to this
must be 1, 2, 3, 4, 5. It must be that they
ate 5 slices of pizza over a total of 8
slices of pizza. So they ate 5/8 of the pizza. You would be right in saying
that they ate 5/8 of the pizza, because they ate 1, 2, 3, 4,
5 out of a total of 8 pieces. But that's not what
this question is asking. They are asking, what fraction
of the pizza is remaining? So what's left over
after Ishaan and Omar had their go at the pizza? Well, what's remaining
is 1, 2, 3 slices. So what's remaining is 3 out
of the original 8 equal parts or 3/8 of the
pizza is remaining. So let's input that. So we could go right here. And we would say 3 over
8, 3/8, is remaining. So let's check our answer,
and we got it right.