The lifetime of a disk drive head is normally distributed with a population mean of 1000 hours and standard deviation of 120 hours. Determine the probability that the average lifetime for 9 disk drives will exceed 940 hours. Need to see work!
Use z-scores:
z = (x - mean)/(sd/√n)
With your data:
z = (940 - 1000)/(120/√9)
Finish the calculation, then determine your probability using a z-table.