ACT Math problem

Question

There are exactly 15 marbles in a jar. Each of these marbles is either solid red or solid blue. Explain why it is not possible for the ratio of red marbles to blue marbles in the jar to be 2:5.

Answer 1

For the ratio of red to blue marbles to be 2:5, the ratio of red marbles to total marbles has to be 2:(2+5) = 2:7

So 2/7 of the marbles would have to be red. But you cant have 2/7 of 15 marbles