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**By Brian Carruthers (Please see notes on slide 1)**

Calculator Tricks 4 By Brian Carruthers (Please see notes on slide 1) HAVE SOMEONE SECRETLY SELECT A THREE-DIGIT NUMBER AND ENTER IT TWICE INTO THEIR CALCULATOR. (FOR EXAMPLE: ) HAVE THEM CONCENTRATE ON THE DISPLAY.YOU WILL THEN TRY TO DISCERN THEIR THOUGHTS! FROM ACROSS THE CLASSROOM, ANNOUNCETHAT THE NUMBER IS DIVISIBLE BY 11.(HAVE THEM VERIFY IT BY DIVIDING THEIR NUMBER BY 11.) ANNOUNCE THAT THE RESULT IS ALSO DIVISIBLE BY 13. HAVE THEM VERIFY IT ON THEIR CALCULATORS. NOW ASK THEM TO DIVIDE THE RESULT BY THEIR ORIGINAL THREE-DIGIT NUMBER. ANNOUNCE THAT THE FINAL ANSWER IS 7. YOU CAN USE THIS TO PREDICT LUCKY 7. IF YOU WANT TO PREDICT UNLUCKY 13, THEN ASK THEM TO DIVIDE BY 7 IN STEP 3; THE FINAL ANSWER WILL BE 13. ENTERING A THREE-DIGIT NUMBER TWICE (123123) IS EQUIVALENT TO MULTIPLYING IT BY (123 X 1001 = 123, 123). SINCE 1001 = 7 X 11 X 13, THE SIX-DIGIT NUMBER WILL BE DIVISIBLE BY 7, 11, 13, AND THE ORIGINAL THREE-DIGIT NUMBER.

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Lucky 7 or Unlucky 13

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**Step One Think of a three-digit number**

Enter it twice on your calculator. For example: Now concentrate on the display.

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**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.

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

Am I right?

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**Step Three But, guess what:**

Your answer displayed on your calculator is divisible by 13. Again, use your calculator to check the answer.

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**7 Now, divide your result by your original three-digit number.**

Your final answer will be : 7

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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.

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The answer will be 13

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**Why does this work? Entering a three-digit number twice**

For example: 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.

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**Watch this: 345345 divided by 11 = 31395 31395 divided by 13 = 2415**

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Now what did you think of that?

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