I want an explanation for this example :

A recent study showed that the amount of sleep per night for students at a college is 8 hrs with a standard deviation of 1.6 hrs. If 64 students were selected at random, what is the probability that their average sleep per night will be less than 7.5 hours? Round answer to two decimals, if necessary. Express answer in percent. Don't forget to put % sign with you answer.