Number game, always ends up the same number

Question

When I was about 12 a man on holiday taught me a mathematical equation that always allows you to guess what the answer is.

It was something along the lines of,

Pick a number between 1 and 10 but dont tell me.

Multiply it by X and divide it by seven. Take a number away and you could guess what the answer was.

I have forgot it cause I last heard it in 1993 or thereabouts. Does anyone know the formula or equation.

Thanks :)

Answer 1

There are a lot of different ways you can do it.  The first one on this page is an example with a phone number:

http://kids.niehs.nih.gov/brnumber.htm

Here are some more examples:

http://www.jimloy.com/puzz/magic.htm

Answer 2

You can make your own. The trick is to multiply by 9 at some point then add up the digits in the number, they'll always add up to 9 (or a multiple of 9 for bigger numbers) and you can wangle it from there.