Eleven

If a number is divisible by 11, reversing its digits will result in another multiple of 11. As long as no two adjacent digits of a number added together exceed 9, then multiplying the number by 11, reversing the digits of the product, and dividing that new number by 11, will yield a number that is the reverse of the original number.

For example:
12 x 11 = 132.
231 / 11 = 21.

Or

142,312 x 11 = 1,565,432. 2,345,651 / 11 = 213,241.

So amazing calculation.