Kindergarten children have heights that are approximately normally distributed about a mean of 39.1 inches and a standard deviation of 1.8 inches.

Kindergarten children have heights that are approximately normally distributed about a mean of 39.1 inches and a standard deviation of 1.8 inches. If a random sample of 20 is taken, what is the probability that the sample of kindergarten children has a mean height of less than 40.5 inches? (Give your answer correct to four decimal places.