Given:

Average value of gas mileage for commuter vans is: 18 mi/gal

Find:

What gas mileage would represent the 90th percentile if the average is 18 mi/gal?

Solution:

So far I have found the corresponding z-score for 90% to be 1.28. However I am having difficulty with the problem because it does not give standard deviation.

All help will be appreciated. Thank you.