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Fraction word problem: piano

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

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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.