How to find a value for the 90% percentile

__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.

Re: How to find a value for the 90% percentile

Hey bob123.

What is the variance or standard error for the mileage? (You need that to get the percentile)

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Re: How to find a value for the 90% percentile

This is why I'm confused. None of that is given, nor does it say to make any kind of assumptions other than gas mileage is assumed to be normally distributed. Here it is straight from the textbook:

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Re: How to find a value for the 90% percentile

You can't figure this out without information for the variance component (population or sample).

You either have to assume it, or measure it.

Re: How to find a value for the 90% percentile

I'll find out of there's a value we're supposed to assume.

Thanks for the information.