## BEATCALC:  Beat the Calculator!

Back to Calculation Tips & Tricks

 From B. Lee Clay

Squaring Numbers

Multiplying Numbers

Dividing Numbers

Subtracting Numbers

Finding Percents

Calculation
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### Multiplying a 2-digit number by 693

1. Select a 2-digit number .
2. Multiply it by 7.
3. If the product is 2 digits, subtract 1:
X X _ _
Then subtract the product from 100: _ _ X X
4. If the product is 3 digits, add one to the first digit and subtract the result from the product:
X X X _ _
Then subtract the last two digits of the product from 100: _ _ _ X X

#### Example:

1. The 2-digit number chosen to multiply by 693 is 14.
2. Multiply by 7: 7 × 14 = 70 + 28 = 98
3. Subtract 1: 98 - 1 = 97: 9 7 _ _
4. 100 - product: 100 - 98 = 2: _ _ 0 2
5. So 14 × 693 = 9,702.

See the pattern?

1. The 2-digit number chosen to multiply by 693 is 87.
2. Multiply by 7: 7 × 87 = 560 + 49 = 609
3. 1st digit + 1: 6 + 1 = 7
4. Subtract 7 from product: 609 - 7 = 602:
6 0 2 _ _
5. 100 - product's last 2 digits: 100 - 9 = 91:
_ _ _ 9 1
6. So 87 × 693 = 60,291.