$21 is 3/4 of $28, because 28 divided by 4 is 7. 7 times 3 is 21. $28 minus $21 is $7. Mel paid $21, and Gretchen paid $7 for the cake. To find the answer, first divide the cost of the cake as a whole by the denominator (bottom or right-side number) of your fraction. Now, take the quotient and multiply it by your numerator (top or left-hand number of your fraction). Subtract your answer from the total cost of the cake, and you’ll see how much was paid. Hopefully this helps!