Question

If light bulbs have lives that are normally distributed with a mean of 2500 hours and a standard deviation of 500 hours, approximately what percentage of light bulbs has a life of more than 3000 hours?

Answer 1

well, 3000 is 1 std above the mean. So, check your Z table for the area of the tail above Z=1.

Or, play around at

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