A particular telephone number is used to receive both voice calls and fax messages. Suppose that 25% of the incoming calls involve fax messages, and consider a sample of 25 incoming calls.

a. What is the probability that at most 4 of the calls involve a fax message?
b. What is the probability that exactly 4 of the calls involve a fax message?
c. What is the probability that at least 4 of the calls involve a fax message?
d. What is the probability that more than 4 of the calls involve a fax message?