申論題

6. You make N phone calls. The length of the ith phone call, denoted by Ti in minutes, is an exponential random variable with expected value 1/λ, where the parameterλ ＞ 0. The random variables T₁, T₂,...,T_N are independent and identically distributed. For any fraction of a minute at the end of a call, the phone company charges for a full minute. In other words, the phone company calculates its charge based on Wi=[T_i]  for the ith phone call. Here the ceiling function of a real number a, denoted by [x], is the least integer number greater than or equal to z. For instance, [3.1] = 4 and [3] = 3.