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Calculator Tricks 4

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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  1. Calculator Tricks 4 By Brian Carruthers (Please see notes on slide 1)

  2. Lucky 7 or Unlucky 13

  3. Step One • Think of a three-digit number • Enter it twice on your calculator. • For example: 123123 • Now concentrate on the display.

  4. 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.

  5. Use your calculator to check that your number is divisible by 11. Am I right?

  6. Step Three • But, guess what: • Your answer displayed on your calculator is divisible by 13. • Again, use your calculator to check the answer.

  7. Now, divide your result by your original three-digit number. Your final answer will be : 7

  8. Watch this: • Enter your original three-digit number twice on your calculator. • Divide that number by 11. • Now divide your result by 7. • Divide your answer by your original three digit number.

  9. The answer will be 13

  10. 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.

  11. 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.

  12. Now what did you think of that?

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