UPSC CAPF Assistant Commandant Model Paper 1

Let the sum of the nos. in the first set be N, and that of the second set be M.
⇒ N/x = y, and M/y = x
⇒ N = xy, and M = xy.
⇒ M + N = 2xy
Since there are x + y numbers, we have
Average = (M+N)/(x+y) = 2xy/(x+y)