Students understand that less than indicates subtraction. So they take away the said amount from the given number. They may count back from the given number to get to the result. In three less within 20 worksheet, students get plenty of practice with this concept. This worksheet is about practicing with the horizontal format in which numbers are written side by side. To develop flexibility with numbers and operations, students need to have enough practice in this format and not just rely on the vertical/column method.