4. The average attention time in the cafeteria of a certain university is 1.5 minutes, assuming that the times of attention follow an exponential calculation distribution:

a) The probability that a student will be served in at least 3 minutes.

b) The probability that a student is served between 1 and 1.6 minutes.

c) What is the maximum attention time of 15% of the cafeteria times? d) What is the variation of service times in the cafeteria?