BEATCALC:  Beat the Calculator!

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Back to Calculation Tips & Tricks
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From the BEATCALC mailing list:
a weekly service from B. Lee Clay, archived here by the Math Forum.
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   Multiplying a 2-digit number by 3 1/2

  1. Select a 2-digit number.
  2. If number is even, divide by 2, then multiply by 7.
  3. If number is odd, multiply by 7, then divide by 2.

   Example:

  1. The 2-digit number chosen to multiply by 3 1/2 is 84.
  2. Divide by 2: 84/2 = 42
  3. Multiply by 7: 42 x 7 = 280 +14 = 294
  4. So 84 x 3 1/2 = 294.

   See the pattern?

  1. The 2-digit number chosen to multiply by 3 1/2 is 63.
  2. Multiply by 7: 63 x 7 = 420 + 21 = 441
  3. Divide by 2: 441/2 = 220.5
  4. So 63 x 3 1/2 = 220.5.


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18 October 1999
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