A mechanic sells a brand of automobile tire that has a life expectancy that is normallydistributed, with a mean life of 30,000 miles and a standard deviation of 2200 miles. He wants to give a guarantee for free replacement of tires thatdon't wear well. How should he word his guarantee if he is willing to replace approximately10% of thetires?

I am doing this one by taking 30000/2200 what = 13.63 then I * it by .10 for 1.3636.