C'mon, Costas, that's an old one.  The trick of, course involves
reversing every
operation - multiply by two,divide by two, add <<number>> subtrct
<<number>>, etc.

Got a better one?

Nick D.
>
> Think of a small number.
>
> Got one ?
>
> OK. Now double your number.
>
> Now add six to it.
>
> Now divide the whole thing in two.
>
> Now take away the number you first thought of ....
>
> Ready ? I now have to guess what you have left ..............
>
> 3 !!
>
> No it's no always three. The reasoning behind this impressive piece of
> complicated math can be obtained by sending me a SAEmail ... :)))
>
> Costas