A set of five transistors is to be tested, one at a time in a random order, to see which of them are defective. Suppose that three of the five transistors are defective. Let n1 denote the number of tests made until the first defective is spotted, and let n2 denote the number of additional tests until the second defective is spotted. Find the probability P(n1 = 1, n2 = 2).
a) 3/20
b) 2/15
c) 1/10
d) 1/15