## 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 7.7

1. Select a 2-digit number.
2. Multiply the number by 7.
3. Move decimal point one place to the left.

#### Example:

1. The 2-digit number chosen to multiply by 7.7 is 21.
2. Multiply number by 7: 7 × 21 = 147.
3. Move decimal one place to the left: 14.7.
4. Add this to 7 times original number (first step answer): 14.7+ 147 = 157 + 4.7 = 161.7.
5. So 7.7 × 21 is 161.7.

See the pattern?

1. The 2-digit number chosen to multiply by 7.7 is 82.
2. Multiply number by 7: 7 × 82 = 560 + 14 = 574.
3. Move decimal one place to the left: 57.4.
4. Add this to seven times original number (first step answer): 57.4 + 574 = 627.4 + 7.4 = 631.4.
5. So 7.7 × 82 is 631.4.
Think left to right when multiplying and adding and these answers should become easy.