Question

How to calculate standard deviation given mean and sample size?

Answer 1

In addition to mean and sample size you will need the individual scores.

The formula for standard deviation is:

S^2 = E(X-M)^2/N-1

Here's an example: 
Data set: 4,4,3,1
Mean: 3
Sample size: 4

First, put the individual scores one after the other and subtract the mean from it.
4 - 3 = 1
4 - 3 = 1
3 - 3 = 0
1 - 3 = -2

Second, square the answers you got from step 1. 
1^2 = 1
1^2 = 1
0^2 = 0
-2^2 = 4
Third, plug the values from step 2 into the formula. 

S^2 = (1+1+0+4)/(4-1) = 6/3 = 2

Standard deviation = 2