I do not understand how to address this issue. Can any one help me, please?

Suppose the number of calls arriving at a telesales call center follows

a fish process of intensity λ = 7 calls per hour. Each caller makes a

purchase with probability p = 0.3.

(i) Find the probability that 4 calls will arrive exactly within the first 90 minutes.

(ii) Suppose that 11 calls arrived within the first two hours. What is the probability that 4 of them have arrived in the first 30 minutes?

(iii) What is the expected time until the first purchase?

(iv) Suppose now that 6 calls arrived in the first hour. What is the probability

that 4 of them have made a purchase?