Find a friend, paper, pencil or calculator.

Choose any single digit number
Example : 9

Multiply it by  3
9 x 3 = 27

Add 12
27 + 12 = 39

Multiply by 15
39 x 15 = 585

Divide by 9
585 / 9 = 65

Multiply by 2
65 x 2 = 130

Divide by 10
130 / 10 = 13

Subtract your original number
13 - 9 = 4

The answer will always be four.

Go out and impress someone!