Between 11 p.m. and midnight on Thursday night, Mystery Pizza gets an average of 4.2 telephone orders

Question:

Between 11 p.m. and midnight on Thursday night, Mystery Pizza gets an average of 4.2 telephone orders per hour. Find the probability that
(a) At least 30 minutes will elapse before the next telephone order;
(b) Less than 15 minutes will elapse; and
(c) Between 15 and 30 minutes will elapse.