# Question Video: Solving Word Problems by Multiplying Fractions by Whole Numbers Mathematics • 5th Grade

Mason ate 3/7 of a cake. If the cake was cut into 21 slices, how many slices did Mason eat?

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### Video Transcript

Mason ate three-sevenths of a cake. If the cake was cut into 21 slices, how many slices did Mason eat?

The first piece of information that we’re given in this problem is the fraction of a cake that Mason ate. And we’re told that he ate three-sevenths of this cake. But we’re also told that the cake was cut into 21 slices. So, really, what we need to do is find the answer to three-sevenths of 21. Now, how can we calculate the value of three-sevenths of 21?

Well, to begin with, we can try to find one-seventh of 21. And we can do that by dividing 21 by seven. How many sevens are there in 21? Let’s count in sevens, seven, 14, 21. There are three sevens in 21. So, 21 divided by seven equals three. And so, we can say that one-seventh of 21 equals three. But, of course, we don’t want to find one-seventh of 21. We want to find three-sevenths of 21.

To do this, we need to multiply one-seventh by three. We know one-seventh is actually worth three. So, all we need to do is to multiply by three by three. And the answer’s nine. Mason ate three-sevenths of a cake. If the cake was cut into 21 slices, we can see that the number of slices that Mason ate is nine. The answer we’re looking for is nine slices.