Adding and Subtracting Tens

Problems such as 30 + 20 and 50 - 20 are handled by counting how many groups of ten there are. Three tens and two tens make five tens, and recounting this way produces the answer, an idea practiced through Calculation, Fill in the Blank and Word Problems.

The first error is answering 5 for 30 + 20, dropping the 0 in the ones place. The opposite also appears, with both zeros lined up to give 500. Whether the answer has two digits shows whether the child is still counting in groups of ten.

Treating 10 as a single unit, an idea taken from Addition and Subtraction up to 20, is stretched here to calculations between tens digits. In 2-Digit ± 1-Digit, ones and tens appear together for the first time, so working with each place separately comes up even more often.