Statistics
posted by Karen on .
The lifetime of a 2volt nonrechargeable 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 ?

http://davidmlane.com/hyperstat/z_table.html
See what happens when you are above the mean by 1.28 standard deviations. 
sadas