AP Statistics

posted by Courtney

Suppose a Normal model describes the fuel efficiency of cars currently registered on your state. The mean is 24 mpg, with a standard deviation of 6 mpg.

-Describe the fuel efficiency of the worst 20% of all cars.

I know that you convert 20% into decimal form to get .2 and put that number in for Invnorm to get -.84, but how do I get the fuel efficiency of the worst 20% of cars from there.

1. Reiny

I don't have a normal curve or a chart handy, so I will trust that your value of -.84 for .2 is correct.

What that means is that 20% lies from the point .84 of a standard deviation to the left of the mean.
.84(6)=5.04

so 20% of the cars will have a fuel efficiency of below 18.96 mpg

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