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This content is taken from the Davidson Institute of Science Education at the Weizmann Institute of Science's online course, An Introduction to Recreational Math: Fun, Games and Puzzles. Join the course to learn more.
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## Davidson Institute of Science Education at the Weizmann Institute of Science

Skip to 0 minutes and 0 seconds Are you ready for your second magic trick? This time I’m going to amaze you, with my fast as lightning brain! Orel is going to help us in this trick as well, right Orel?

Skip to 0 minutes and 11 seconds Yossi: Orel has a calculator with her. Now I’m going to show you how I can do some lightning calculations in my head. Do you believe that?

Skip to 0 minutes and 20 seconds Yossi: OK!! I’m going to show you. I want you to… …choose any number ending in a 5, so, 15, 125, whatever.

Skip to 0 minutes and 31 seconds Yossi (repeats): 45 So what I’m going to ask you to do, when I say “Go!” is to try and square the number on your calculator, that is, do 45 times 45. Before you tell me the answer I’m going to tell you the answer, OK?

Skip to 0 minutes and 45 seconds Yossi: Go! 45 squared is 2025! (repeats) 2025

Skip to 0 minutes and 50 seconds Orel: Oh my God! True?

Skip to 0 minutes and 52 seconds Yossi: Let’s try another one then… Give me another number. Any number ending in 5.

Skip to 0 minutes and 58 seconds Yossi: 85. Thats… go on square it… 7225!

Skip to 1 minute and 5 seconds Orel: whoa! How do you do that?

Skip to 1 minute and 9 seconds Yossi: You wish you knew, ha?