MathXplosion

Pick a Block of Dates - Any Block

A trick you can do by calculating the sum of a 3-by-3 block of dates on a calendar without using a calculator. This episode introduces the concept of the mean (or average): the number in the middle square is the mean of the nine numbers that form the 3 X 3 square. If you add all the numbers and divide by nine (the number of squares) the answer is the number in the central position. So, to find the sum of the nine numbers quickly, just multiply the number in the middle by 9.