[wastholm]

I don't know if it qualifies as a "mathematical coincidence" or not, but I find it amusing to demonstrate to kids that you can multiply 142,857 by 1, 2, 3, etc., and the result is always a number with the same digits but in a different order:

2 × 142,857 = 285,714

3 × 142,587 = 428,571

4 × 142,587 = 571,428

Etc. But then, suddenly:

7 × 142,857 = 999,999

[carrolldunham]

Some kids might love that but I wonder if it's also the kind of thing that makes many kids resent math and ask why do we have to learn this. Since its just like nerd party trick that reveals nothing about the world and in fact makes the world especially math, seem even more impenetrable.

[captn3m0]

There's actually some interesting math behind it: https://en.wikipedia.org/wiki/Cyclic_number, https://www.youtube.com/watch?v=WUlaUalgxqI (Numberphile)

[xtiansimon]

You’re not wrong. The difference (and this does not apply here in an open forum) must be the quality of teaching. Strange math tricks by themselves are empty calories.