The amount of daily time that teenagers spend on a brand A cell phone is normally distributed with a given mean mu is equal to 2 point 5 hr and standard deviation sigma is equal to 0 point 6 hr. What percentage of the teenagers spend more than 3 point 1 hr?

The value 3.1 hours is one standard deviation above the mean: z = (3.1 − 2.5) / 0.6 = 1. In a normal distribution, about 16% of observations lie above a z-score of 1, so about 16% of teenagers spend more than 3.1 hours on the phone.

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