## 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 6.4

1. Select a 2-digit number.
2. Multiply number by 4.
3. Move decimal point one place to the left.
4. Add this to 6 times original number.

#### Example:

1. The 2-digit number chosen to multiply by 6.4 is 21.
2. Multiply number by 4: 4 × 21 = 84.
3. Move decimal one place to the left: 8.4.
4. Add to original number: 8.4 + 6(21) = 8.4 + 126 = 134.4.
5. So 6.4 × 21 is 134.4.

See the pattern?

1. The 2-digit number chosen to multiply by 6.4 is 72.
2. Multiply number by 4: 4 × 72 = 280 + 8 = 288.
3. Move decimal one place to the left: 28.8.
4. Add six times original number: 28.8 + 6(72) = 28.8 + 420 + 12 = 40.8 + 420 = 460.8.
5. So 6.4 × 72 is 460.8.
Multiply and add left to right and you will be able to give these answers quickly.