If a set of observations is normally distributed, what percent of these differ from the mean by A mechanic sells a brand of automobile tire that has a life expectancy

that is normally distributed, with a mean life of 31,000 miles and a standard deviation of 2200 miles.He wants to give a guaranteé for free replácement of tires that don't wear well. How should he word his guarantee if he is willing to replace approximately 10% of the tires?