The number of phone calls coming into a switchboard in the next five minutes will either be 0, 1, or 2.?

Question

The probabilities are the same for each of these (1/3). If X is the number of calls arriving in a five-minute time period, what is the mean of X?

Answer 1

You need the sum of (outcome)*(probability of that outcome); in this case you have (0)(1/3) + (1)(1/3) + (2)(1/3) = 1.