Astudy indicates that 18- to 24- year olds spend a mean of 140 minutes watching video on their smartphones per month. assume that the amount of time watching video on a smartphone per month is normally distributed and that the standard deviation is 15 minutes. complete parts (a) through (d) below. a. what is the probability that an 18- to 24-year-old spends less than 120 minutes watching video on his or her smartphone per month? the probability that an 18- to 24-year-old spends less than 120 minutes watching video on his or her smartphone per month is nothing. (round to four decimal places as needed.)