The amount of time taken by a bank teller to serve a randomly

selected customer has a normal distribution with a mean of 2 minutes and a

standard deviation of 0.5 minute.

a) What is the probability that both of two randomly selected customers will take

less than 1 minute each to be served?

b) What is the probability that at least one of four randomly selected customers

will need more than 2.25 minutes to be served?