Solving Confidence Intervals

first, calculate the standard deviation:

6.4, 6.6, 6.7, 6.8, 7.1, 7.2, 7.6, 7.7, 7.7, 7.7

the standard deviation is: 0.48 (obtained from a calculator)

you haven't provided the % of confidence, so I will assume it's 90%

the value of n = 10, so use n - 1 = degrees of freedom being 9.



use this table for the chi values



since we want 90%, then we use 95% and and 5%, always use the next one greater than it

>> 3.325 is for 95%

>> 16.919 is for 5%

using the formula above:

[ (10 - 1)*0.48^2 ] / 16.919 < σ^2 < [ (10 - 1)*0.48^2 ] / 3.325

0.12 < σ^2 < 0.62

0.34 < σ < 0.79

So we are 90% confident that the standard deviation for bank A is between 0.34 and 0.79 minutes.

Good luck