Video: Calculating a Fraction of an Integer Number in Word Problems

1/5 of the 45 students in a cooking class are studying to be chefs. How many students is this?

02:35

Video Transcript

One-fifth of the 45 students in a cooking class are studying to be chefs. How many students is this?

We’re told that there are 45 students in a cooking class. And we need to calculate the number of them that are studying to be chefs, which is one-fifth of them. So, we need to find one-fifth of 45. We know that to find one-fifth of a number, we need to divide by five. Let’s see why that is. Instead of writing one-fifth of 45, we can write one-fifth multiplied by 45.

And to help us multiply the two together, we need to write 45 as a fraction. We can write 45 as 45 over one. This is the same as 45. Now, we can just multiply the numerators and the denominators as if we were multiplying normal fractions. One times 45 equals 45. And five multiplied by one equals five. So, we could say the answer to our question is forty-five fifths.

But that’s not really a very good way of saying the number of students that want to study to be chefs. So, we need to think about forty-five fifths as a division. It stands for, or represents, 45 divided by five, which is exactly what we said we needed to do to find a fifth of 45. This bar represents the number of students in the class, 45. And by dividing this number by five, we’ll be able to work out what one-fifth is worth.

So, how many fives are in 45? Well, here, we can use our knowledge of times tables to help us. We know that 10 times five equals 50. And so, 45 is only one less five than this. So, instead of 10 times five, 45 is worth nine times five. So, we can rearrange our multiplication to turn it into a division. 45 divided by five equals nine. And so, we can say one-fifth of 45 is nine. And so, the number of students that are studying to be chefs in the cooking class is nine students.

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