Calculator Tricks 4

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# Calculator Tricks 4 - PowerPoint PPT Presentation

Calculator Tricks 4. By Brian Carruthers (Please see notes on slide 1). Lucky 7. or Unlucky 13. Step One. Think of a three-digit number Enter it twice on your calculator. For example: 123123 Now concentrate on the display. Step Two. I think that your number is divisible by 11.

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## Calculator Tricks 4

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Presentation Transcript

### Calculator Tricks 4

By

Brian Carruthers

(Please see notes on slide 1)

### Lucky 7

or

Unlucky 13

Step One
• Think of a three-digit number
• Enter it twice on your calculator.
• For example: 123123
• Now concentrate on the display.
Step Two
• I think that your number is divisible by 11.
• Do you believe me?
• If you don’t then I’ll prove it to you.

### Use your calculator to check that your number is divisible by 11.

Am I right?

Step Three
• But, guess what:
Watch this:
• Divide that number by 11.
• Now divide your result by 7.

13

Why does this work?
• Entering a three-digit number twice
• For example: 123123 is the same as multiplying it by 1001.
• 123 x 1001 = 123,123.
• Because 1001 = 7 x 11 x 13, the six-digit number will be divisible by 7, 11, 13, and your original three-digit number.
Watch this:
• 345345 divided by 11 = 31395
• 31395 divided by 13 = 2415
• 2415 divided by 345 = 7
• 678678 divided by 11 = 61698
• 61698 divided by 7 = 8814
• 8814 divided by 678 = 13.

of that?