The life span of the battery pack is known to be Normally distributed with a mean of 250 hours and a standard deviation of 20 hours. How to find the percentage of the battery packs lasting more than 250 hrs.
Z = (score-mean)/SD
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. Convert to percentage.
However, since this value is the mean, the mean = median = mode in a normal distribution. Median = 50th percentile.