Standard deviation一問

Given that X is the mean of three positive numbers p,q and r,and the following are two sets of data.

A: p,q and r.
B:p/2,q/2,r/2

(a) Find the mean of set B (in terms of X).
(b) If a number of X are added in set A such that the standard deviation of set A is just equal to that of set B, how many data are therein set A now?