As part of its quality assurance program, the Autolite Battery Company conducts tests on battery life. For a particular D-cell alkaline battery, the mean life is 19 hours.

The useful life of the battery follows a normal distribution with a standard deviation of 1.2 hours.

a) About 68 percent of the batteries failed between what two values?

b) About 95 percent of the batteries failed between what two values?

c) Virtually all of the batteries failed between what two values?