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.