Multiply a multi-digit sequence of numbers
                 by 8, add a number, subtract 20

1. Choose a number made up of consecutive digits beginning with 1 (123, 1234 etc. - up to 9 digits).
2. Multiply it by 8.
3. Add the last digit of the original sequence. The answer will be a declining sequence beginning with 9 and the same length as the original sequence.
4. Subtract 20.

   Example:

1. If the number selected is 123:
2. Multiply by 8, add 3 (descending sequence beginning with 9 and 3 digits long): 987
3. Subtract 20: 987 - 20 = 967
4. So 123 multiplied by 8 plus 3 - 20 is 967.

   See the pattern?

1. If the number selected is 123456:
2. The answer before the last subtraction is 987654 (descending sequence from 9 with 6 digits)
3. Subtract 20: 987654 - 20 = 987634
4. So 123456 multiplied by 8 + 6 - 20 is 987634.

Extend the number of examples by changing the last step - or subtract or add different numbers.