√n[Sigma(f * x^2)] - [Sigma(f*x]^2

_____________________________ = s

n(n-1)

I get

√ [29(11213.9)] - (523.5)^2

______________________

29(28)

So,

325203.1 - 274052.25

___________________

812

51151

_____ = √62.99

812

And then I get 7.93

I got all the results from:

Classes----Frequency ---- Midpoint (x) ---- x^2

10-13 ---- 1 -----11.5------132.25

14-17 ---- 0 -----15.5 ----- 240.25

18-21 ---- 15-----19.5-----380.25

22-25 ---- 7------23.5-----552.25

26-29 ---- 2------27.5-----756.25

And then of course:

x^2*f

132.25

0

5703.75

3865.4

1512.5

Sigma=11213.9

The 523.5 I got from Sigmaf*x