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 5 5/7

  1. Multiply the number by 4.
  2. Add a zero.
  3. Divide by 7.

   Example:

  1. If the 2-digit number chosen to multiply by 5 5/7 is 32:
  2. Multiply by 4: 4 x 32 = 128
  3. Add a zero: 1280
  4. Divide by 7: 1280/7 = 182 6/7
  5. So 32 x 5 5/7 = 182 6/7.

   See the pattern?

  1. If the number to be multiplied by 5 5/7 is 61:
  2. Multiply by 4: 4 x 61 = 244
  3. Add a zero: 2440
  4. Divide by 7: 2440/7 = 348 4/7
  5. So 61 x 5 5/7 = 348 4/7.

Numbers divisible by 7 will produce whole-number answers. With other numbers, those using calculators will get long, inexact decimals.


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6 March 2000
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