In the biathlon event of the Olympic Games, a participant skis cross-country and on four intermittent occasions stops at a rifle range and shoots a set of five shots.

If the center of the target is hit, no penalty points are assessed. If a particular man has a history of hitting the center of the target with 90% of his shots, what is the probability of the following?

a. He will hit the center of the target with all five of his next set of five shots.

b. He will hit the center of the target with at least four of his next set of five shots. (Assume independence.)