The lifetime of a certain brand of battery is normally distributed with a mean value of 8 hr and a standard deviation of 0.8 hr when it is used in a particular cassette player. Suppose that two new batteries are independently selected and put into the player. The player ceases to function as soon as one of the batteries fails. (a) What is the probability that the player functions for at least 6 hr?
(b) What is the probability that the cassette player works for at most 9 hr?