***Before I ask, my question got booted and I'm not sure why. I'm studying for a TEAS exam for nursing entrance and I am not taking as statistics or probability course. However, this question came up in a study manual and I'm having a hard time learning the material.

Consider the case in which each of two piles initially contains fifteen marbles, where one pile contains fifteen white marbles and the other pile contains fifteen black marbles. Suppose that on the first transfer, three black marbles are moved to the white pile. On the second transfer, any three marbles are taken from the white pile and put into the black pile. Demonstrate, with diagrams and words that you will always end up with as many white marbles in the black pile as black marbles in the white pile.