Need to know which formulae which has been used

[zorro]

Question :

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?

[maggiemagnet]

If you're not sure what formula this is, try working backwards from the "sample" formulas they've given you. Which one uses in the numerator? Which one also uses in the denominator?

[zorro]

If you're not sure what formula this is, try working backwards from the "sample" formulas they've given you. Which one uses in the numerator? Which one also uses in the denominator?

Thanks!!