First the 3.1 hr is one standard deviation away from the mean: 2.5 + 0.6 = 3.1 hr. then the one standard deviation away from the mean contains 68% of the total population. so the percent that is higher from 3.1 hr is % higher 3.1 hr = ( 100 - 68) / 2 % higher 3.1 hr = 32 / 2 % higher 3.1 hr = 16 %