4.21. Suppose that the weekly use of gasoline for motor travel by adults in North America is approximately normally distributed, with a mean of 16 gallons and a standard deviation of 5 gallons.

(a) What proportion of adults use more than 20 gallons per week?

(b) Assuming that the standard deviation and the normal form would remain constant, to what level must the mean reduce so that only 5%

use more than 20 gallons per week?

(c) If the distribution of gasoline use is not actually normal, how would you expectit to deviate from normal?