If the population mean payment time is 19.5 days, what is the probability of observing a sample mean payment time of 65 invoices less than or equal to 13.107? days? In a normally distributed random variable, it is easier to calculate the probability of the value that is occurring in the interval of the means. This is possible through the standardization of the normally distributed random variable. 'With standardization, it helps in the determination of the probability that is associated with a range of values for the variable. In the formula below, letter Z represents the standardized random variable as well as the probabilities that are linked with the ranges of Z and can be traced in the Z distribution table. Z value for 18.1077 is z = {18.107719.5}f0.5209 = -2.57 P [mean X