The print on the package of 100-watt General Electric soft-white

light-bulbs says that these bulbs have an average life of 750 hours. Assume that

the lives of all such bulbs have a normal distribution with a mean of 750 hours and

a standard deviation of 55 hours. Find the probability that the mean life of a random

sample of 25 such bulbs will be

a) greater than 735 hours,

b) between 725 and 740 hours,

c) within 15 hours of the population mean,