A store sells three types of pens: blue, black, and red. The probability of a customer buying a blue pen is 0.4, a black pen is 0.35, and a red pen is 0.25. If a customer buys two pens, what is the probability that they buy one blue pen and one black pen?
Since the customer buys two pens, we can consider this as two separate events. Let's denote the first event as E1 (buying the first pen) and the second event as E2 (buying the second pen).

The probability of buying one blue pen and one black pen can occur in two ways:
1. Buying a blue pen first (E1) and then a black pen (E2)
2. Buying a black pen first (E1) and then a blue pen (E2)

We need to find the probabilities for both cases and add them together:

Case 1: P(E1 = Blue) * P(E2 = Black) = 0.4 * 0.35 = 0.14

Case 2: P(E1 = Black) * P(E2 = Blue) = 0.35 * 0.4 = 0.14

Total probability = 0.14 + 0.14 = 0.28

So, the probability that a customer buys one blue pen and one black pen is 0.28.