And since I'm easily amused and kind of a math geek...here's how the number is always four...
Let x be the number you choose. No restrictions on the domain.
1. Think of a number.
x
2. Multiply it by two.
2x
3. Add eight to the product.
2x + 8
4. Divide the total by two.
(2x + 8) / 2
5. Subtract the first number (x).
(2x + 8)/2 - x
6. Simplify.
(2x + 8)/2 - x =
(2/2)x + (8/2) - x=
(x + 4) - x = 4