The mean lifetime of a certain tire is 30,000 miles and the standard deviation is 2500 miles.

Question:

The mean lifetime of a certain tire is 30,000 miles and the standard deviation is 2500 miles.
a. If we assume the mileages are normally distributed, approximately what percentage of all such tires will last between 22,500 and 37,500 miles?
b. If we assume nothing about the shape of the distribution, approximately what percentage of all such tires will last between 22,500 and 37,500 miles?