[VanceWorks]

I have three numbers 12.1g, 12.4g, and 12.6b.

means equal total of three number divided by 3.
answer: 12.4g

average deviation each number minus the means add together then divided by 3.
answer: -.033g

standard deviation squareroot  then each number minus the means then squared add together then divided by 3-1.
answer: .255g

I think this is how you get the answer??

[sdekivit]

standard deviation is given by:

sqrt sigma (X - X(mean))^2 / n - 1