Answer:
He should word his statement as "Free replacement for tires that wear before 30875 miles."
Step-by-step explanation:
If he is willing to replace 10% of tires he should find the life of tires that gives an area of 10% in the normal distribution graph.
Now for 10% of area standard normal deviate Z can be obtained from normal distribution table
Using normal distribution table for 10% area we have Z = -1.28
Thus we have
Applying given values we get