Finding the combined standard deviation

Suppose i have 2 sets

Set A: number of items = 10, mean = 2.4 , S.D = 0.8

Set B: number of items = 5, mean = 2, S.D = 1.2

Ok i can find the combined mean easily, but how am i supposed to find the combined standard deviation?

Any help would be greatly appreciated...

Thanks

Re: Finding the combined standard deviation

1. Var(A+B) = Var(A) + Var(B) ( Given that both observations are independent; Otherwise, Var(A+B) =/= Var(A) + Var(B) )

2. 1.2 square + 0.8 square = 2.08 (this is the combined variance)

3. square root 2.08 = 1.442 (combined standard deviation)