Addition and Subtraction up to 20

Using the idea of ten and some more, this unit covers addition and subtraction up to 20 with no carrying and no borrowing, such as 13 + 4 and 17 - 5. The group of ten stays as it is and only the ones digits are worked on, and Fill in the Blank and Word Problems confirm the same idea.

Writing 13 + 4 as 7 is the telling error: only the ones are added and the group of ten is dropped. A single-digit answer points to this pattern. Answering 7 for 17 - 5, by subtracting from the 1 in the tens place as well, follows close behind.

The reasoning itself does not change; only the range of numbers grows beyond Addition up to 10 and Subtraction up to 10. Extending the range while leaving the underlying way of seeing numbers untouched is the same path taken again in Adding and Subtracting Tens.