Lesson One: Determining a person’s age 
[Aug. 1st, 200607:30 pm]
Arithmancy: HP math geeks unite

The age of a person is needed for many advanced Arithmantic spells. Many witches and wizards are aware of this fact and, not understanding Arithmancy, are reluctant to give their age for fear that their ignorance will be exploited. (Arithmantic ethics are studied in year two, so we won’t spend any time on the subject here.) This lesson demonstrates a simple spell for determining the age of a person. They will never actually state their age aloud, and they need never know that you have learned it, so it is quite useful.
The Spell
 Have the subject write his or her age on a piece of paper, and not to show it to you.
 There are twelve months in a year, so ask the person to add 12 to their age.
 There are seven days in a week and fiftytwo weeks in a year, so ask the subject to add their result to 752.
 Ask the subject to give you only the last digit of the total.
From this information, you can now determine the age of the person by subtracting four from the digit. (You may need to add ten you can “borrow”.) The result is the actual last digit of the person’s age. You should be able to determine what the first digit should be by looking at the person and guessing.
How does this work?
Suppose the person's age is 27. In steps 2 and 3, you have them add 12 and 752 to their age. But notice that they have only added 4 to the ones digit of their age. This allows you to simply subtract 4 from their given last digit to get the actual ones digit of their age. 

