please solve and explain

Prob. 3 (23 points) It is conjectured that an impurity exists in 25% of all drinking wells in a certain rural community. In order to gain some insight into the true extent of the problem, it is determined that some testing is necessary. It is too expensive to test all of the wells in the area, so 15 are randomly selected for testing. (a) What is the probability that between 2 and 5 wells (inclusive) have the impurity? (b) What is the probability that the number of impure wells is more than its mean value plus one standard deviation