If four marbles are picked at random, what is the probability that at least one is blue?

Probability that none is blue from four marbles
= ^{15 - 4}C₄ / ¹⁵C₄
= ¹¹C₄ / ¹⁵C₄ = 22/91
So, probability that at least one is blue from four marbles = 1 - 22/91 = 69/91