## 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

Full List

### Multiplying a 2-digit number by 5.7

1. Multiply the number by 7.
2. Move decimal point one place to the left.
3. Add to 5 times the original number.

#### Example:

1. The first number to multiply by 5.7 is 21.
2. Multiply by 7: 7 × 21 = 147.
3. Move decimal point one place to the left: 14.7.
4. Add to 5 times the original number: 14.7 + 5(21) = 14.7 + 100 + 5 = 114.7 + 5 = 119.7.
5. So 5.7 × 21 = 119.7.

See the pattern?

1. The second number to multiply by 5.7 is 46.
2. Multiply by 7: 7 × 46 = 280 + 42 = 322.
3. Move decimal one place to the left: 32.2
4. Add to 5 times the original number: 32.2 + 5(46) = 32.2 + 200 + 30 = 62.2 + 200 = 262.2.
5. So 5.7 × 61 = 262.2.

Multiplying and adding left to right is the key to producing these answers using mental math.