Take a calendar. Draw a square around any nine dates. (It will be a 3 x 3 square, right?) Let be the smallest number in the square. If you add up all the numbers in the square, the answer will always be a multiple of 9. In particular, the sum will be exactly .
Why?
Solution
A 3x3 square of dates from a calendar will look like the figure below, where is the smallest date in the square. Add everything up in the square and you’ll get Voila!
