What Happens: The class is shown a grid of consecutive numbers. Without you looking at the grid, a student circles any number and then crosses out all other numbers in the row and column of the selected number. This process continues until the last number is circled. At that time all the students find the sum of the circled numbers. Again, without looking, you will then determine the sum of the circled numbers by giving an oral mixed combination for all the students to solve, something like 64 ÷ 8 x 9 + 7 – 5 = ? The answer to the mixed combination will be the sum of the circled numbers.
The Challenge: After doing this trick with five different number grids over a period of time, the students then try to figure out how you are able to determine the sum of a set of unknown numbers.
This resource includes:
- instructions detailing the procedure of this activity, including an explanation and demonstration of the secret
- slideshow with 5 number grids (1 – 16, 1 – 25, 1 – 36, 1 – 49, 1 – 64) in 2 formats – Adobe PDF and Microsoft PowerPoint 2010. These slides can be projected to do this activity with the whole class.
- slideshow with black-and-white slides that can be printed to make a hard copy of this activity (also in 2 formats). The printed copies of the grids can be used in individual or small group situations.