Math Mentalist - The Commutative Law on Steroids!

Grade Levels
6th - 12th, Higher Education, Adult Education
Formats Included
  • PDF
22 pages
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Showmanship recommended, but not necessary. This demonstration will leave your students stunned. Guaranteed to amaze and delight! They will learn a mathematical property which they will never forget. This demonstration works in a classroom of 30 or in the auditorium in front of the whole student body. It's perfect for a substitute teacher who needs to hold the class' attention. A daunting task when in front of new students who know you'll be there only once.

The teacher, in full view of his class, takes a blank piece of paper and writes a number on it. Only he sees the number. He takes a 9 by 12 manila envelope, demonstrates that it is empty and slides the paper into it. Everyone watches as he bends the clasps, sealing the paper in. He then asks a trusted student to take the envelope and return to his desk with the instructions that no one is to tamper with it. Absolutely no one!

He then reveals three piles of slips of paper which for easy reference will be called "Red Pile, "Green Pile" and "Blue Pile". He explains to the class that each Pile has 15 slips of paper with a four digit number written on them in the color of their heading.

The Stage is set. The 45 slips of paper are randomly dispersed among the students. An irrefutable check proves no numbers are alike. A randomly selected student now picks one student from each colored slip group. The three remaining students are asked to stand. They each randomly select numbers from their slip which are written in three columns on the blackboard. A true random event has been created! The teacher draws a line under the last row of numbers and adds the columns.

He says aloud the final number and asks the trusted student holding the sealed envelope to come to the front of the room with him. He asks him to open the envelope. The student does. Just before he takes out the paper in it, the teacher holds up his hands and ask for the entire class to say aloud the number written on the blackboard. He wants that number reverberating in the room. They do. He nods his head for the trusted student to continue. The student unfolds the paper and reads the numbered inscribed in a loud voice.

It's the same!

The students are aghast. The teacher sits weary from the mental strain.

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
Total Pages
22 pages
Answer Key
Teaching Duration
45 minutes
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