Answer always comes 1089

1.  Take any three digits number in which digits on hundreds  place is greater than digits on ones place.  
2.  Write the number in reverse order.
3.      Subtract the number of step II from the number of Step I.
4.    Add the reverse number of step III to the number of step III itself. Then, answer always comes 1089.

e.g.  

 1) Any number is           563

2) The reverse of 563 is     -365

3) Difference                198

4) Reverse of 198 is         +891

5) Ans:                    1089

Reason:

1)      Difference between a three  digits number in which digit on hundreds place is greater than digit on ones place  & its reverse is always multiple of 99.

2)      Sum of result of (1) in three digits number & its reverse is always 11 times 99, i.e. 1089.