To evaluate the performance of a certain brand of alkaline battery, researchers at a consumer testing laboratory measure the lifetime of 160 1.5-volt batteries. Battery lifetime for this study is defined as the time it takes the voltage to drop to 1.0 V under a standard electrical load. The researchers determine that the mean lifetime is 48.3 hours with a standard deviation of 15.7 hours.
Assuming a normal distribution of battery lifetimes,
(a) Find the probability that a battery's voltage will drop below 1.0 V after 20 hours of use.
(b) Find the probability that a battery's voltage will drop below 1.0 V after 70 hours of use.
(c) If the daily production rate of 1.5-V batteries is 25,000, how many batteries per day will have lifetimes of 20 hours or less and 70 hours or more?