Why is it always 1089

(This is the reasoning behind Always End With 1089)

WE will need a little bit of 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−1 B+9 C+10
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