please answer question with (b)

You are working for a telephone company and you are asked to design a land-line network for a new residential area. The network is circuit switched. Since it is a new area, traffic statistics are not available. There are three approximations of the traffic rate: 30 calls/minute, 20 calls/minute, and 50 calls/minute. Each of the approximations has a probability of 0.3, 0.2 and 0.5, respectively. The duration of calls has an average of 3 minutes. (b) Design the number of circuits such that the blocking probability is 0.02 or less