the life span of an electric bulb is normally distribution with mean 5500 light hours and standard deviation of 1200 light hours . what is the probability that a randomly selected bulb which lasts more than 6000 light hours?

Answer 1

well, 6000 is 5/12 std above the mean.
Look that up in your Z table.

You can play around with this stuff at

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