Question

The useful life of a football used by high school students is normally distributed with a standard deviation of 5 hours. The probability that a football is replaced before 20 hours of use is 0.0150. What is the average lifetime of a football?

Answer 1

the answer is 30.85, i doubt it. i got 9.17

Answer 2

use the table on page 6 to see that P(Z < -2.17) = 0.0150

So the z-score is z = -2.17

Use this to find the mean mu

z = (x-mu)/sigma

-2.17 = (20 - mu)/5