If a company were to buy 1500 of these lightbulbs how many may burn out before burning for 830 hr

Question

Value Watt lightbulbs have an average lifeitme of 750 hr of use before burning out, with a standard dviation of 80 hr, according to the quality control department.  Assume these times are normally distributed.

Answer 1

Z=(830-750)/80=1. This Z value corresponds to a probability of 0.8413. 830 hours is one standard deviation away from the mean. I interpret this as 0.8413 of 1500 bulbs are likely to fail before burning for 830 hours. This works out to be 1262 failures in 1500.