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 875

  1. Select a 2-digit number.
  2. Multiply by 7.
  3. Divide by 8.
  4. Move the decimal point 3 places to the right.

   Example:

  1. The 2-digit number chosen to multiply by 875 is 38.
  2. Multiply by 7: 7 x 38 = 210 + 56 = 266
  3. Divide by 8: 266/8 = 33 2/8 = 33.25
  4. Move the decimal point 3 places to the right: 33250
  5. So 875 x 38 = 33250.

   See the pattern?

  1. The 2-digit number chosen to multiply by 875 is 74.
  2. Multiply by 7: 7 x 74 = 490 + 28 = 518
  3. Divide by 8: 518/8 = 64 6/8 = 64.75
  4. Move the decimal point 3 places to the right: 64750
  5. So 875 x 74 = 64750.


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17 May 1999
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