7. The lifetime of a certain brand of tires is approximately normally distributed, with a mean of 40,000 miles and a standard deviation of 2,500 miles under normal driving conditions.

Tire wear is greatly affected by road, weather and driver conditions along with proper

maintenance of the tires. A driver who aggressively accelerates or makes quick stops, for

example, will wear out a tire much more quickly. The brand carries a warranty of 33,000 miles

under normal driving conditions, i. e., the company will replace a tire if it wears out before this

mileage limit is reached.