A study from the University of Michigan, as noted in Newsweek (March 25, 2002), stated that men average 16 hours of housework each week

(up from an average of 12 hours in 1965). If we assume that the number of hours in which men engage in housework each week is normally distributed with a standard deviation of 5.4 hours, what is the probability that the mean number of housework hours for a sample of 20 randomly selected men is between 15 to 18 hours?