Statistics
posted by jacob .
The lifetime of a certain smart phone battery has an
unknown distribution with mean value of 8 hours and
standard deviation of 2 hours. What is the approximate
probability that the average battery lifetime of a sample
of 36 batteries will exceed 8.1 hours?

Assuming a normal distribution,
Z = (scoremean)/SEm
SEm = SD/√n
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to the Z score.