Lesson 2: Passing a secret number to a group 
[Aug. 1st, 200607:25 pm]
Arithmancy: HP math geeks unite

In this lesson, you will learn to cast a spell that will force all who listen to think of the same number. I should note that it is possible to perform a similar feat with the use of the Imperius curse, but the Arithmantic techniques for doing so are easier. Not to mention far more legal.
The Spell
 Ask your subjects to think of any even number.
 Tell them to double this number, and then add 12 to the total.
 Ask your subjects to divide the new total by 4.
 Finally, tell your subjects to take half of the original number and subtract it from the last total.
The number everyone is now thinking of is 3.
How does this work?
First, note that any even number can be written as two times another number, for example 34 can be written as 2×17. So whatever number the person wrote down, we can write it as 2N.
The second step asks the subject to double the number and add 12: 2×2N + 12, or 4N + 12.
The third step asks the person to divide their number by 4, which gives N + 3.
And finally, when you ask the subjects to take half of the original number and subtract it, note that what you are actually asking them to do is subtract N. So (N + 3)  N = 3. Thus, no matter what number they start with, they will always get 3. 

