Amazing mind-reading math trick.

[cannabis_flower]

18. that's hardly an amazing mystery!

[(x times 3) +3] times 3 = (3x+3)times 3 = 9x +9 which is divisible by 9.

Any number you choose you will get a number divisible by 9.

If you chose 1:

9(1)+9 =18. Add the digits together and you get 9
9(2)+9 =27 2+7=9
9(3)+9=36 3+6=9
9(4)+9=45 4+5=9
9(5)+9=54 5+4=9
9(6)+9=63 6+3=9
9(7)+9=72 7+2=9
9(8)+9=81 8+1=9
9(9)+9=90 9+0=9

If you add the digits of any number together and they add up to 9, the number is divisible by 9. Same goes for 3. If you add up all the digits and they are divisible by 3, the number is divisible by 3.