The company wishes to offer a warrantee for a free replacement of any transmissions that fail prematurely, but only wants to replace 1% of the transmissions that they sell. How long should they set the warrantee period?

I have just reread the question, with one above. There is more information...

It has been determined that the lifetime of transmission is normally distributed with an average of 78,000 miles and a standard deviation of 15000 miles

This is the whole question. It has been determined that the lifetime (time until failure) of this transition is normally distributed with an average of 78,000 miles and a standard deviation of 15,000 miles.

First question: What percentage of these transmissions will last at least 90,000 miles? answer= 21%

So the second question..

So the company wishes to offer a warrantee for a free replacement of any transmissions that fail prematurely, but only wants to replace 1% of the transmissions that they sell. How long should they set the warrantee period?