II. Rapid addition

Have the spectator draw a 4 X 4 square around any sixteen dates. Ask the spectator to use a calculator to add up all of the numbers within the square. You then rapidly calculate the sum of all of those dates by applying the following formula:

Take the lowest number and add 8

Multiple the above result by 9

4 Square Calendar

In the above example the sum of all of the numbers in the indicated square would be 90 since 8 +2 = 10 and 10 X 9 = 90