a)The probability that no sale is made on the first three calls is the product

of the prob that no sale is made on the first and the prob that no sale is

made on the secons and the prob that no sale is made on the third.

So the reqired prob is (1-0.19)^3 ~= 0.531

b) The probability that the fouth sale is made on the eighth call is the

probability of 3 sales on the first seven calls and a sale on the eighth.

So the reqired prob is b(3;7,0.19) 0.19 = 7!/[4! 3!] 0.19^3 (1-0.19)^4 0.19 ~= 0.0196.

(as the nu,ber of sales in N calls has a binomial distribution B(7,0.19)

RonL