The random variable X denotes the time taken for a computer link to be made between the terminal in an executive's office and the computer at a remote factory site. It is known that X has a normal distribution with mean 15 seconds and a standard deviation 3 seconds. P(X>20) has value.
Z for this condition is Z=(20-15)/3 = 1.67. The probability that Z is greater than this i.e. that X>20 is p=.04746 from a standard table of the cumulative normal distribution.