Date: Jan 17, 2006 6:21 AM
Author: Patricia Murphy
Subject: Calendar Math Trick
A recent issue of Big Learning News mentioned the following trick. Note: Check out the original source that was cited, i.e. a page from the MurderousMaths web site, for an illustrated example, and for other tricks:
<<Math Moment Calendar Trick
This trick will let kids impress their friends while giving their mental multiplication and addition skills a workout. On any month's calendar page, tell the friend to draw a square around any three-by-three square of dates on the page. Instantly tell them the sum of those 9 numbers.
To do the trick, you take the middle number of the square and multiply it by 9. To do that, multiply by 10 and subtract the middle number. For instance, if you draw a 3-by-3 square starting on Sunday, January 1, 2006, the sum of the dates is 1 + 2 + 3 + 8 + 9 + 10 + 15 + 16 + 17, which equals 81. The middle square is January 9, and 9 x 9 indeed equals 81!>>