Problem 136
Battery Power Problem. A certain type of thermal battery for an airplane navigation device backup power has a mean life of 300 hours with a standard deviation of 15 hours.
What proportion of these batteries can be expected to have lives of 322 hours or less? Assume a normal distribution of backup power device lives.
Solution
Z = 
= 1.47
P(z<1.47)= 0.9292
so 92.92% of all batteries can be expected to live 322 hours or less.