A study 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 har smartphone per month is ]. (Round to four decimal places as needed.)