The lifetime of a 2-volt non-rechargeable battery in contant use has a normal distribution with a mean of 516 hours and a standard deviation of 20 hours. Ninety percent of all batteries have a lifetime less than ?

Answer 1

http://davidmlane.com/hyperstat/z_table.html

See what happens when you are above the mean by 1.28 standard deviations.

Answer 2

sadas