Hey earthboy.
Do you know results regarding variances for random variables?
(Hint: things like Var[aX + bY] = a^2*Var[X] + b^2*Var[Y] if X and Y are independent)
If the frequency ,mean and standard deviation of two sets of observations be denoted by and respectively, then prove that where, and are the mean and standard deviation of the combined set of observations.
thanks in advance!