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## 4th grade

### Unit 8: Lesson 3

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

Sal solves a fraction word problem about practicing piano. The fractions have common denominators. .Created by Sal Khan.

## Video transcript

Pedro is supposed
to practice piano for 3/4 of an hour every day. Today, he has practiced
for 1/4 of an hour. What fraction of an hour
does he need to practice? So let's visualize 3/4. So he needs to practice
3/4 of an hour. So if this represents
an entire hour, so let's divide it into fourths. So now, divide it into halves. Now, let's make it into
four equal sections. Now, it's into fourths. So he needs to practice
3/4 of an hour. That was my attempt
at drawing an arrow. So let me shade it in. So he needs to practice
3/4 of an hour. This gets us to 3/4
right over here. So he needs to practice 3 out
of the fourths of an hour. So this is 3/4. Now, it says he has already
practiced for 1/4 of an hour. So he's already
practiced for 1/4. So how much more does
he need to practice? Well, he needs to
practice this much more. And you might already
see the answer visually. But let's think about
how to represent it as a fraction expression. So let me write it like this. So this is how much
he needs to go. How much does he
need to practice? So he needs to do
a total of 3/4. He's already done 1/4. So if you subtract
1/4 from the 3/4, you're going to get this
amount right over here. You're going to get the amount
that he needs to practice. Now, this already has
the same denominator. So this is going to be
equal to 3 minus 1 over 4, which is equal to 2/4. And you see that
right over there. He still needs to practice 1
and 2 out of the fourths, 2/4. Now, 2/4, we know, is
the same thing as 1/2. There's a couple of
ways to think about it. You could say,
hey, look, this is half of the length
of this entire thing. It has a little bit on
this end and that end. Or if you make 4 equal
blocks, and if you were to shade in
2 of them, you see that you have shaded
in 1/2 of the blocks. This is the exact same
amount as if I just divide it into 2 sections and I
shaded in only 1 of them. 2/4 is the same thing as 1/2. And if you wanted to work
it out mathematically, you just have to
do the same thing to the numerator
and the denominator. So let's divide the
numerator and the denominator by 2, because they are
both divisible by 2. That's actually their
greatest common factor. So 2 divided by 2 is 1. 4 divided by 2 is 2. So what fraction of an hour
does he need to practice? He's got to practice
1/2 an hour.