Okay, this was taught to me on the last day of school by a student named Maria, so I'll call it "Maria's Magic Math"
Okay, you tell the class to get their calculators out and you are going to prove you can get the answer to the sum of FIVE 5 digits numbers faster then them!
Ask a student for a 4 digit number, write it on board, and then say, "Oops, I said 5 digits numbers, didn't I? So I'm going to add a 2 on the end" and ofcourse add the number TWO to the end so now its a five digit number.
Ask a second student for a 5 digit number and write it directly underneath.
Now you say that you are going to supply a 5 digit number. Looking at the number the student just gave you, under each digit you write the number that would add to 9, so if the first digit is a 3 you write a 6.
After you have written your number, skip down a little and draw a line that the answer would be under. Look at the board for a minute and act out some "concentrating and thinking" and then announce you know what the answer is going to be and write the number 2, then the first 4 numbers the student gave you and then a zero.
Ofcourse they don't have 5 numbers on their calculator to add up yet!
So now ask another student for a 5 digit number and write it on board.
You volunteer the last 5 digit number, using method above where your digits add to 9 with the digits above them!
Now they have all five 5 digit # to add and see that your answer was right!
Thanks Maria!
1 comment:
Nifty... I'll have to try it. The only thing that makes it a little suspicious is that you write the answer before you get the numbers so you could easily figure out what the last number would have to be to make your number add up to what you want. It seems fixed without them even knowing the method behind it.
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