Regain of parent numbers .

Regain of parent number.
First take two three digit numbers like 123 & 789.
    Let, 123 be x and 789 be Y. and do following steps.

step 1:-=[(x+y)-(y-x)]/2
           = x
   then, answer will get smaller no. or 'x' .
    EXAMPLE.-
1]  =[(123+789)-(789-123)]/2
      =[912-666]/2
      =[246]/2
      =123=x

step 2:- =[(x+y)+(y-x)]/2
           = y
   then, answer will get smaller no. or 'y' .
    EXAMPLE.-
2]  =[(123+789)+(789-123)]/2
      =[912+666]/2
      =[1578]/2
      =789=y

 *Therefore, we get our parent numbers      again