Posted by Anonymous on .
The numbers of hours of life of a torch battery is normally distributed with a mean of 150 hours and standard deviation of 12 hours. In a quality control test, two batteries are chosen at random from a batch. If both batteries have a life less than 120 hours, the batch is rejected. Find the probability that the batch is rejected.

statistics 
PsyDAG,
Z = (scoremean)/SD = (120150)/12
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to that Z score.
The probability of both/all events occurring is found by multiplying the individual probabilities.
I hope this helps.