.  Leaking gas tanks. Leakage from underground gasoline tanks at service stations can damage the environment. It is estimated that 25% of these thanks leak. You examine 15 tanks chosen at random. Independently of each other. 
(a) what is the mean number of leaking tanks in such samples of 15? 
(b) what is the probability that 10 or more of these 15 tanks leak? 
(c ) now you do a larger study, examining a random sample of 1000 tanks nationally. What is the probability that at least 275 of these tanks are leaking?