maths-lover

You will need a little bit off algebra.
But stick with it, it is not difficult.
Represent the number with ABC
Reverse this to get CBA
Remember that A is Hundreds B is tens and C is units
Take CBA from ABC like this
Hundreds Tens Units

A B C

C B A
Now here is the trick: subtract 1 Hundred, and add 9 Tens and 10 Ones (-100, +90, +10 = 0, so won't change answer):

Hundreds Tens Units

A-1A B+9B 10+CC

C B A

A-1-C 9 10+C-A
Last Step: Reverse the answer and add the two numbers together.

A-1-C 9 10+C-A
+
10+C-A 9 A-1-C

9 18 9
(simplify:)
10 8 9
As predicted the answer was 1089


Note:But sometimes I ended with 198 not 1089. This happens if you pick a number like 546. Try to pick consecutive numbers like 123, 345, 765 etc. If it does happen, it is not a problem. Just repeat the stages again starting from 198.