We are starting with 8+4+7+5=24 marbles. We're taking out 8, so we'll be left with 24-8=16 marbles. Think about those 16. If the probability of drawing a white one is 1 in 4, that means that 1/4 of the marbles are white. 1/4 of 16 is 16/4 = 4 marbles are white. Since we started with 8 white, and there are 4 left, we must have taken out 8-4 = 4 white marbles.
