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

1. Select a 2- or 3-digit number.
2. Divide by 4 (or by 2 twice).
3. Move the decimal point one place to the right, or add a zero.

#### Example:

1. If the 2-digit number chosen to multiply by 2 1/2
is 98
2. Divide by 2: 98/2 = 49
3. Divide by 2: 49/2 = 24.5.
4. Move the decimal point one place to the right: 245
5. So 98 × 2 1/2 = 245.

See the pattern?

1. If the 3-digit number chosen to multiply by 2 1/2
is 123
2. Divide by 2: 123/2 = 61.5
3. Divide by 2: 61.5/2 = 30.75.
4. Move the decimal point one place to the right: 307.5
5. So 123 × 2 1/2 = 307.5.

1. If the 3-digit number chosen to multiply by 2 1/2
is 488
2. Divide by 4: 488/4 = 122
3. Divide by 2: 61.5/2 = 30.75.
4. Move the decimal point one place to the right (add a zero): 1220.
5. So 488 × 2 1/2 = 1220.

Divide by 4 when it is easy (the number is divisible by 4 if its last two digits are divisible by 4), or divide by 2 twice. This method should become an easy one.