## BEATCALC:  Beat the Calculator!

Back to Calculation Tips & Tricks

 From B. Lee Clay

Squaring Numbers

Multiplying Numbers

Dividing Numbers

Subtracting Numbers

Percents

Calculation
Practice Exercises

Full List

### Subtract from a repeating 1's number, divide              by 9, subtract 21

1. Select a number between 2 and 10 digits made up of 1's.
2. Subtract a number equal to the number of digits in the number you have selected.
3. Divide by 9. The answer will have one less digit than the original number in the series 123456789.
4. Subtract 21.

#### Example:

1. If the repeating number selected has 5 digits:11111:
2. Subtract 5 (same as number of digits) and divide by 9. The answer will be 1234 (4 digits).
3. Subtract 21: 1234 - 21 = 1213.
4. So (11111-5)/9 - 21 = 1213.

See the pattern?

1. If the repeating number selected has 8 digits:11111111:
2. Subtract 8 (same as number of digits) and divide by 9. The answer will be 1234567 (7 digits).
3. Subtract 21: 1234567 - 21 = 1234546.
4. So (11111111-8)/9 - 21 = 1234546.

For those who enjoy extensions of these patterns:
(answers after the division by 9, step 3)

11 digit number, answer is 1234567900
12 digit number, answer is 12345679011
13 digit number, answer is 123456790122
14 digit number, answer is 1234567901233
15 digit number, answer is 12345679012344

and ?

Extend the exercises using this basic pattern by changing step 4 to add or subtract other numbers.