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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   Dividing a 2-digit number by 7 1/2

  1. Select a 2-digit number.
  2. Multiply it by 4 (or by 2 twice).
  3. Divide by 3.
  4. Move the decimal point 1 place to the left.

   Example:

  1. The 2-digit number chosen to multiply by 7 1/2
    is 42.
  2. Multiply by 4: 42 x 4 = 168
  3. Divide by 3: 168/3 = 56
  4. Move the decimal point 1 place to the left: 5.6
  5. So 42 divided by 7 1/2 = 5.6.

   See the pattern?

  1. The 2-digit number chosen to multiply by 7 1/2
    is 93.
  2. Multiply by 4: 93 x 4 = 360 + 12 = 372
  3. Divide by 3: 372/3 = 124
  4. Move the decimal point 1 place to the left: 12.4
  5. So 93 divided by 7 1/2 = 12.4.

Practice and you will soon be cranking out these quotients with speed and accuracy.


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27 April 1998
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