A calculator is perfectly reliable except for two parts: its microchip, which has a failure rate of one in every twenty hours of operation; and its battery, which has a failure rate of one in every ten hours of operation. Also, on average the battery will fail in five hours, with a standard deviation of 30 minutes. What service life should the manufacturer specify in order to have a .8413 probability that a battery will not fail within that amount of time?

Select one:

a. 5 hours

b. 4.5 hours

c. 5.5 hours

d. 4 hours

e. 3.5 hour