The number of hours adults sleep per night is normally distr

The number of hours adults sleep per night is normally distributed with a mean of 7 hours. Assume that the standard deviation is unknown. If 75% of adults sleep more than 6.5 hours per night, what is the variance? (Remember the label.)

Solution

1 - 0.75 = 0.25

z = -0.67

-0.67 = (6.5 - 7 ) / sigma

sigma = 0.7463