TLF ID M025010
Learn a cool trick using the concept of the mean (or average). Pick any 3 x 3 block of dates on a monthly calendar. 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 the total 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.