I used to be confused when I see the word standard deviation and thought of skipping the question... but now it is easy:
Let's say the set of numbers is {a,b,c,d,e,f} which has 6 numbers
1- get the average of the set of numbers = a+b+c+d+e+f/6
2- (average-a)^2 + (average-b)^2 + (average-c)^2 + (average-d)^2 + (average-e)^2 + (average-f)^2 / 6 = VARIANCE
3- Standard Deviation= square root of Variance
THAT'S IT ^___^
by a little practice it will take less than a minute
http://www.mathsisfun.com/data/standard-deviation.html