Two-fifths equals eight what.
Here, we have two equivalent fractions. This means that they’re worth the same. But the second of our two fractions is missing a denominator. Two-fifths is the same as eight what? To find a fraction that’s equivalent to another fraction, we either need to multiply the numerator and the denominator or we need to divide them. But whichever we choose, we need to do the same thing to both numbers. This way the value of the fraction will stay the same. So are we going to multiply? Or shall we divide?
Well, we’re given a big clue as to what to do because we can see how the numerator changes, we’re given the numerator in the second fraction. In the first fraction, it’s two and it becomes eight. We can say then that the number two has been multiplied. And it’s been multiplied by four. Two times four equals eight. To keep the fraction worth the same then, we need to multiply the denominator by four, too. Five times four equals 20. So the missing denominator is 20. Two-fifths is equivalent to eight-twentieths. Our missing denominator is 20.