(40+38+36+28+20+17+14+10+9.7+8.5) / 10 = 221.2/10 = 22.12
(40 - 22.12)^2 = 319.6944
(38 - 22.12)^2 = 252.1744
(36 - 22.12)^2 = 192.6544
(28 - 22.12)^2 = 34.5744
(20 - 22.12)^2 = 4.4944
(17 - 22.12)^2 = 26.2144
(14 - 22.12)^2 = 65.9344
(10 - 22.12)^2 = 146.8944
(9.7 - 22.12)^2 = 154.2564
(8.5 - 22.12)^2 = 185.5044
added = 1382.396
(sum of differences) / (10 - 1) = 1382.396 / 9 = 153.5995556 ... this is ur variance
standard deviation is square root of variance...
sqrt (153.59995556) = 12.39 <== thats rounded
