Each problem gives a fraction and a second fraction with the same value but a larger denominator and a blank numerator. The student works out what number the top must be. It is the step before adding fractions with unlike denominators, where finding a common denominator is exactly this.
Whatever you multiply the denominator by, you multiply the numerator by the same. 2/3 with a denominator of 12 means multiplying by 4, so the numerator is 8.
Two fractions that represent the same amount, like 1/2 and 3/6. You get one from the other by multiplying or dividing the top and bottom by the same number.
See what the denominator was multiplied by, then multiply the numerator by the same number.
To add 1/3 and 1/4 you first rewrite both over 12 - which is finding equivalent fractions. This page practises that step on its own.
Third and fourth grade, before adding and subtracting fractions with unlike denominators.