Hi
My recommendation would be an Excel electronic worksheet
0r something similar. Key is: (12.5-8.6)/2 = 1.95 and 8.6+1.95 = 10.55, for ex
x n xn x -17.43 n(x -17.43)^2
10.55 13 137.15 -6.88 615.3472
14.55 16 232.8 -2.88 132.7104
18.55 5 92.75 1.12 6.272
22.55 4 90.2 5.12 104.8576
26.55 12 318.6 9.12 998.0928
50 871.5 5.6 1857.28 (sum)
mean 17.43 37.90367347 Variance sum/(n-1)
6.156595932 SD (sqrt of variance)