The Joy Of 9

Adding the digits of a multiple of 9 always gives a multiple of 9. For example: 9 x 4 = 36, and 3 + 6 = 9. Inmodular ari... (more)

Adding the digits of a multiple of 9 always gives a multiple of 9. For example: 9 x 4 = 36, and 3 + 6 = 9. Inmodular arithmetic, this property allows checking answers by "casting out nines." A related trick: mentally computing the day of the week for any date in history. (less)