Multiplying a 4-digit number by
137, then
by 73; subtract 1200

Choose a 4-digit number (one whose numbers are all the same).

Multiply by 137 and multiply the product by 73.

The second product is an 8-digit number: the original number and a repeat of
the digits.

Subtract 1200.

Example:

If the first number is 6241:

The answer to the two multiplications is 62416241

Subtract 1200: 62416241 - 1200 = 62415041

So 6241 × 137 × 73 = 62,415,041.

See the pattern?

If the first number is 7834:

The answer to the two multiplications is 78347834

Subtract 1200: 78347834 - 1200 = 78346634

So 7834 × 137 × 73 = 78,346,634.

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.