Subtraction up to 20

Taking the 1 in the tens place as something to subtract from as well shrinks the answer down to a single digit. Running a finger along the equation shows whether the number being taken away is aimed at the ones alone. In Fill in the Blank, the two numbers get subtracted even where the box stands at the far left, and those items alone come out wrong.

Only the sign has changed; splitting a number by place, the thing to keep an eye on, is shared with Sums up to 20. Calculation inside twenty is complete with these two lessons, and Adding and Subtracting Tens goes on to move the tens digit itself.