What is the probability that the average lifetime of 25 bulbs would be no more than 1550 given mean of 1500 and standard deviation of 200 hours.

I know that it is asking P(x less than or equal to 1550), x is 25, u equal 1500 and sd is 200. I'm having problems getting to the final answer.
Thank YOu!

1 answer

poke around here, and I think all will become clear:

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