I feel the need to offer you encouragement. Though the statistical formulas and symbols may be unfamiliar to you, if you follow the examples and plug in the correct numbers in the formula, you'll do super!
To compute the standard deviation when scores (interval level data) are in a frequency distribution use the computational formula and set up the following column headings:
x, f , fx , x2 , fx2
The variance, s2 , is arrived at first: s2 = SS/n-1; SS stands for the sum of the squared differences.
SS = E fx2 - (E fx)2 /N where E is the symbol for sum of.
After you attain the SS, divide it by n-1. Then you'll have the variance. Take the square root of the variance and you'll have the standard deviation.