An urn contains 75 white balls and 150 black balls. Next to the urn is a large pile of black balls.
Again and again, someone removes a pair of balls from the urn. If either ball is black, then a black ball is placed on the pile, and the other ball is returned to the urn. If both balls are white, then they are discarded, and a black ball is taken from the pile and returned to the urn.
On every turn, there is one less ball in the urn. What color is the last marble in the urn?