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,

d) less than the population mean by 20 hours or more.