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
