There's 5 marbles that are yellow. There's 15 in total, so he has a 5 out of 15 chance to pick a yellow marble, "probability" is basically "What is the chance?".
Your answer is D. Because in the denominator in the ratio, 15, is added up with all of the same numbers of a ratio. Your numerator is the purple and yellow marbles added up. Its really simple.
