Customers arrive at a supermarket according to a Poisson process with rate

12 per hour. Compute the probabilities that

(a) 4 customers arrive between 9:00 and 9:40.

(b) 15 customers arrive between 9:40 and 11:20.

(c) 4 customers arrive between 9:00 { 9:40 and 15 arrive between 9:40 { 11:20.

(d) 4 customers arrive between 9:00 { 9:40 or 15 arrive between 9:40 { 11:20.