BEATCALC:  Beat the Calculator!

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Back to Calculation Tips & Tricks
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From B. Lee Clay


 Squaring Numbers

 Multiplying Numbers

 Dividing Numbers

 Adding Numbers

  1. even numbers 2 through selected 2-digit evens
  2. digits of square of repeating ones
  3. consecutive odds
  4. consecutive between 2 numbers
  5. sequence from 1 to selected 2-digit number
  6. sequence from 1 to selected 1-digit number and back
  7. sequences in the 10's
  8. sequences in the 20's
  9. sequences in the 30's
  10. sequences in the 40's
  11. sequences in the 50's
  12. sequences in the 60's
  13. sequences in the 70's
  14. sequences in the 80's
  15. series of doubles
  16. series of quadruples
  17. series of 10 numbers

 Subtracting Numbers

  1. 1's repeating, divide by 9, subtract 21
  2. 8's repeating, divide by 9, subtract 10
  3. squares of two numbers
  4. reversing/adding/subtracting 3-digit numbers

 Percents

  1. finding 2.5 percent
  2. finding 5 percent
  3. finding 15 percent
  4. finding 20 percent
  5. finding 25 percent
  6. finding 33 1/3 percent
  7. finding 40 percent
  8. finding 45 percent
  9. finding 55 percent
  10. finding 60 percent
  11. finding 70 percent
  12. finding 75 percent

Calculation
Practice Exercises

Full List



  Subtract from a repeating 1's number, divide
             by 9, subtract 21

  1. Select a number between 2 and 10 digits made up of 1's.
  2. Subtract a number equal to the number of digits in the number you have selected.
  3. Divide by 9. The answer will have one less digit than the original number in the series 123456789.
  4. Subtract 21.

   Example:

  1. If the repeating number selected has 5 digits:11111:
  2. Subtract 5 (same as number of digits) and divide by 9. The answer will be 1234 (4 digits).
  3. Subtract 21: 1234 - 21 = 1213.
  4. So (11111-5)/9 - 21 = 1213.

   See the pattern?

  1. If the repeating number selected has 8 digits:11111111:
  2. Subtract 8 (same as number of digits) and divide by 9. The answer will be 1234567 (7 digits).
  3. Subtract 21: 1234567 - 21 = 1234546.
  4. So (11111111-8)/9 - 21 = 1234546.

    For those who enjoy extensions of these patterns:
    (answers after the division by 9, step 3)

    11 digit number, answer is 1234567900
    12 digit number, answer is 12345679011
    13 digit number, answer is 123456790122
    14 digit number, answer is 1234567901233
    15 digit number, answer is 12345679012344

    and ?

Extend the exercises using this basic pattern by changing step 4 to add or subtract other numbers.


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26 August 1996
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