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 two 2-digit numbers
            (difference of 2)

  1. Choose a 2-digit number.
  2. Select a number either 2 smaller or 2 larger.
  3. Square the average of the two numbers.
  4. Subtract 1 from this square.

   Example:

  1. If the first number is 29, choose 31 as the second number.
  2. The average of 29 and 31 is 30. Square 30: 30 x 30 = 900.
  3. Subtract 1: 900 - 1 = 899.
  4. So 29 x 31 = 899.

   See the pattern?

  1. If the first number is 76, choose 74 as the second number.
  2. The average of 76 and 74 is 75. Square 75: 75 x 75 = 5625.
  3. Subtract 1: 5625 - 1 = 5624.
  4. So 76 x 74 = 5624.

    Practice this shortcut (remember your methods of squaring numbers).


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25 March 1996
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