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.
Show the number as ABC
Reverse this to get CBA
Remember that A is hundreds B is tens and C is ones
Take CBA from ABC like this:
| Hundreds | Tens | Ones | |
| A | B | C | |
| Subtract: | C | B | A |
| A−C | 0 | C−A |
Now here is the trick:
Subtract 1 hundred, and add 9 tens and 10 ones
(−100, +90, +10 = 0, so won't change the answer):
| Hundreds | Tens | Ones | |
| A−1 | B+9 | C+10 | |
| Subtract: | C | B | A |
| A−1−C | 9 | 10+C−A |
Last Step: Reverse the answer and
add the two numbers together.
| Hundreds | Tens | Ones | |
| A−1−C | 9 | 10+C−A | |
| Add: | 10+C−A | 9 | A−1−C |
| 9 | 18 | 9 | |
| Simplify: | 10 | 8 | 9 |
As predicted the answer was 1089