Find the arithmetic mean and the standard deviation of each frequency distribution.

Class Limit Frequency

1-5 2

5-9 8

9-13 15

13-17 6

17-21 38

21-25 31

25-29 13

29-33 7

I managed to find the arithmetic mean doing 2320/120 giving me 19.33

But the standard deviation is giving me trouble

I know the formula is (x-mean)^2 times F

and I did that for example (3-19.33)^2=531.8 (3 being the class mark)

I did that for all them and added them up to get 5040

Then I did stat dev= square root (5040/100) and I get 7.0009 but checking my answer apparently it is wrong.

Is there any way I can do this with a calculator?