The amount of daily time that teenagers spend on a brand a cell phone is normally distributed with a given mean mc027-1.jpg = 2.5 hr and standard deviation mc027-2.jpg = 0.6 hr. what percentage of the teenagers spend more than 3.1 hr?

First, we get the z-score by using the equation,
                                   z-score = value - mean / standard deviation
Substituting,
                                    z-score = (3.1 - 2.5) / 0.6 = 1
Converting the z-score to percentage will give us 0.841. Subtracting this value from 1.0 and multiplying the difference by 100%.
                      percentage = (1 - 0.0841) x 100% = 15.9%
Thus, 15.9% of the teenagers spend more time in the cellphone.