The lifetime of a certain smart phone battery has an

Question

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?

Answer 1

Assuming a normal distribution,

Z = (score-mean)/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.