## 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- or 3-digit number by 1 2/3

1. Select a 2-digit number.
2. Divide by 6 (or by 2 and 3).
3. Move the decimal point one place to the right, or add a zero.

#### Example:

1. The 2-digit number chosen to multiply by 1 2/3 is 78.
2. Divide by 2: 78/2 = 39
3. Divide by 3: 39/3 = 13. (39 is divisible by 3 because the sum of its digits is divisible by 3.)
5. So 78 × 1 2/3 = 130.

See the pattern?

1. If the 3-digit number chosen to multiply by 1 2/3 is 315:
2. Divide by 3: 315 / 3 = 105. (315 is divisible by 3 because the sum of its digits is divisible by 3.)
3. Divide by 2: 105/2 = 52.5.
4. Move the decimal point one place to the right: 525.
5. So 315 × 1 2/3 = 525.

Test the number to see if it is divisible by 3 (if the sum of the digits is divisible by 3, so is the number). If not, divide by 2 first. This should make the division easier.