A snow tire manufacturer believes that a typical set of snow tires lasts on average for 30,000 miles. They also believe that 95% of drivers get between 20,000 and 40,000 miles of use from the tires.

A snow tire manufacturer believes that a typical set of snow tires lasts on average for 30,000 miles. They also believe that 95% of drivers get between 20,000 and 40,000 miles of use from the tires. What value of σ, the standard deviation, would be needed to make the information above approximately consistent with a Normal distribution model for tire wear? You should use the Empirical Rule to answer this question.