Question

if you had a rectangular sheet cake do you need to cut into 15 pieces how many pieces would have to be on each side to achieve that many servings out of the whole cake

Answer 1

15 is divisible by 3 and 5.
3 * 5 = 15.
You can cut the cake into 5 pieces along the length, and cut into 3 pieces along the width to get 15 pieces.