## 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 repeating 3's number by 15

1. Choose a repeating 3's number (3333, etc.)
2. Multiply it by 15
3. The product is:
- the first digit is 4
- next repeating 9 digits -- one fewer than the repeating 3's in the original number
- the last digit is 5

#### Example:

1. If the first number is 33333:
2. The product is:
- the first digit is 4: 4.
- next repeating 9 digits -- one fewer than the repeating 3's in the original number: 9999.
- the last digit is 5: 5.
3. So 15 × 33333 = 499995.

See the pattern?

1. Next pick is 33333333:
2. The product is:
- the first digit is 4: 4.
- next repeating 9 digits -- one fewer than the repeating 3's in the original number: 9999999.
- the last digit is 5: 5.
3. So 15 × 33333333 = 499999995.

http://mathforum.org/

15 April 2008
Web page design by Sarah Seastone