Multiplying a 5-digit number by 9091, then
                  by 11; subtract 1200

1. Choose a 5-digit number.
2. Multiply by 9091 and multiply the product by 11.
3. The second product is a 10-digit number: the original number and a repeat of the digits.
4. Subtract 1200.

   Example:

1. If the first number is 34986:
2. The answer to the two multiplications is 3498634986
3. Subtract 1200: 3498634986 - 1200 = 3498633786
4. So 34986 x 9091 x 11 = 3,498,633,786.

   See the pattern?

1. If the first number is 22854:
2. The answer to the two multiplications is 2285422854
3. Subtract 1200: 2285422854 - 1200 = 2285421654
4. So 22854 x 9091 x 11 = 2,285,421,654.

You can expand this exercise by changing the final step to another calculation: add or subtract a number that is easy to handle. In this way you can create many new exercises.