Magic Number for Manipulating Mind

With this magic trick, there could manipulate the mind 5000 or more spectators in the same time.

Note: this magic trick is used for opening, and you could say that "I will share a mind-reading trick that will be done by you all (the audience) at home."

Procedure
1. Say that you will try to manipulate the minds of an entire audience that there are with numbers (If necessary provide a blackboard to write down your instructions so that it can be seen by the audience). You can say "Please select any number from 0 to one billion, even up to one trillion, but try to figure that simple, because we will play with a bit count. "
2. Instruct the audience for multiplying two numbers that were chosen earlier.
3. Then added with a six.
4. For two
5. Reduce that number by the number who first thought.
6. It must be the result 3!

The key is in point no.3. Whatever number you requested to add, certainly a result half of the figure were added. Suppose you are asked to add 8, surely the end result 4. In order not to seem like a formula, after step 5, you can ask a spectator to name the date of birth, and ask the entire audience to add up the numbers they get with those birthdate. It must be the result remains the same.

Note: Although this trick seems so simple and like "toys of elementary school children", but it is effective to win the sympathy the audience at the beginning of the show.