How to be a Mathematical Clairvoyant
Requirements: Spooky music and spooky look.
You will find some spooky music here.
Tell your students you will beat them adding up 5 x 5 digit numbers in your head when they are using calculators.
Method:
1. Ask a student to write down 2 x 5-digit numbers on the board.
2. You rapidly write a 5-digit number underneath.
3. Ask another student to write another 5-digit number.
4. You write another 5-digit number quickly.
5. You have 5 by 5 digit numbers. Say ‘Go’. You instantly write down the answer.
Stand back.
This is how it works:
N1 = 97413
N2 = 28619
N3 = 71380 (Each digit in N3 that you write down must add up to 9 with digits in No. above)
N4 = 64231
N5 = 35768 (Once again each digit in N5 must add up to 9 with digits above)
Now you will instantly write down the sum of these five numbers as
297411
Da! DA!
The trick is to subtract 2 from N1 and put it in front:
N1 = 97413
N1 -2 = 97411
Sum of 5 numbers = 297411
This is why it works:
Hint: When you get your students to add up the five 5-digit numbers on a calculator you will beat them, but they will also get many different answers as a number of students will key incorrect numbers.
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