## BEATCALC:  Beat the Calculator!

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 From B. Lee Clay

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### Multiplying a 2-digit number by 3.9

1. Move decimal one place left.
2. Multiply original number by 4.
3. From this product subtract first answer.

#### Example:

1. The 2-digit number chosen to multiply by 3.9 is 21.
2. Move decimal one place left: 2.1.
3. Multiply original number by 4: 4(21) = 84.
4. From this product subtract first answer: 84 - 2.1 = 82 - 0.1 = 81.9.
5. So 3.9 × 21 is 81.9.

See the pattern?

1. The 2-digit number chosen to multiply by 3.9 is 74.
2. Move decimal one place left: 7.4.
3. Multiply original number by 4: 4(74) = 280 + 16 = 296.
4. From this product subtract first answer: 296 - 7.4 = 289 - 0.4 = 288.6.
5. So 3.9 × 74 is 288.6.

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