The tread life a certain brand of tires is known to be normally distributed with a mean of 60,000 miles and a standard deviation of 5,000 miles. The company wishes to set the warranty so that only 4% of the tires will need to be replaced. What should the warranty mileage be?

Answer 1

4% lie 1.75 std below the mean.
So, 60000-5000*1.75 = 51250

If the warranty is for 51250, 96% of the tires will exceed that mileage.

You can enter your values and play around with Z table stuff at

http://davidmlane.com/hyperstat/z_table.html