Andyat11
It should work all the time, if not, it is that you're not doing the correct math, or the guy did it on purpose to make it feel "more realistic".
Schmidte :
Definitely, we were not at the same school, I'm not talking about the ability to do additions.
The bottom of the "trick" is that any digit multiplied by 9, will give you either a one or two digits number, take the sum of those individual digits and it will give you 9 again.
With that principal, you can do any trick involving the number 9.
1 * 9 = 9 Now : 9
2 * 9 = 18 Now : 1+8=9
3 * 9 = 27 Now : 2+7=9
4 * 9 = 36 Now : 3+6=9
5 * 9 = 45 Now : 4+5=9
6 * 9 = 54 Now : 5+4=9
7 * 9 = 63 Now : 6+3=9
8 * 9 = 72 Now : 7+2=9
9 * 9 = 81 Now : 8+1=9
10 * 9 = 90 Now : 9+0=9
That's the "9 mistery", again, learned in elementary schoool. My elementary school, sorry.