The amount of daily time that teenagers spend on a brand a cell phone is normally distributed with a given mean of 2.5 hr and standard deviation of 0.6 hr. what percentage of the teenagers spend more than 3.1 hr?

5%
10%
16%
32%

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 %

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