Statistics
posted by Pat .
A tire manufacturer wishes to set a minimum mileage guarantee on a new set of tires. The mean is 67,900 with a standard deviation of 2,050. The manufacturer wants to set a minimum guaranteed mileage so that no more than 4% of the tires will have to be replaced. What should the minimum mileage be?

71490 miles nearly.
http://davidmlane.com/hyperstat/z_table.html
Use the second applet, put in mean, std deviation, set above to .04