2-Digit ± 1-Digit

This unit practices adding or subtracting in the ones place while the tens place stays untouched. It splits into addition without carrying and subtraction without borrowing, and gathers only forms such as 24 + 3 and 38 - 5, where nothing crosses into the next place.

Answering 54 for 24 + 3 shows a place-value mix-up, with the single-digit number added to the tens. A changed tens digit in the answer identifies this pattern. In Word Problems, children sometimes add the two numbers when asked what is left, producing an answer with too many digits.

This unit combines the tens-place calculation from Adding and Subtracting Tens with work in the ones place. In Calculations with 3 Numbers, it is not the place value that grows but the count of numbers, from two to three, and the practice turns to working through single-digit numbers one after another.