## BEATCALC:  Beat the Calculator!

Back to Calculation Tips & Tricks

 From B. Lee Clay

Squaring Numbers

Multiplying Numbers

Dividing Numbers

Subtracting Numbers

Finding Percents

Calculation
Practice Exercises

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### Multiplying a 2-digit number by 69 1/2

1. Select a 2-digit number.
2. Divide by 2.
3. Subtract this from 70 times the original number.

#### Example:

1. The 2-digit number chosen to multiply by 69 1/2 is 21.
2. Divide by 2: 21/2 = 10 1/2.
3. Subtract from 70 times the original number: 70(21) - 10 1/2 = 1400 + 70 - 10 1/2 = 1470 - 10 1/2 = 1460 - 1/2 = 1459 1/2.
4. So 21 × 69 1/2 = 1459 1/2.

See the pattern?

1. If the 2-digit number chosen to multiply by 69 1/2 is 83:
2. Divide by 2: 83/2 = 41 1/2.
3. Subtract from 70 times the original number. 70(83) - 41 1/2 = 5600 + 210 - 41 1/2 = 5810 - 41 1/2 = 5770 - 1 1/2 = 5770 - 1 1/2 = 5769 - 1/2 = 5768 1/2.
4. So 83 × 69 1/2 = 5768 1/2.
Practice multiplying and subtracting left to right. Then you will be competent in doing these computations mentally.

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16 August 2004
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