The average working-set size X of a program is normally distributed with the unknown mean and a known variance

^{2}=81 . The program was executed 36 times and the average working set size for each run recorded. The sample mean was computed to be 100 page frames . Assuming successive runs of the program are independent, find the 95% confidence interval for the mean average working set size

Solution:

(100-1.96 * 9/6 , 100 + 1.96 * 9/6) = (97.06 , 102.94)

Need to know which formula been used in order to get this result?