There are approximately one billion smartphone users in the world today. In the United States, the ages 13 to 55 of smartphone users approximately follow a normal distribution with an approximate mean and standard deviation of 36.9 years and 13.9 years, respectively. Forty percent of the ages that range from 13 to 55 are at least what age?
a) 30.5 years
b) 35.2 years
c) 40.0 years
d) 43.7 years
