3-Number Calculations

Working three numbers from left to right is consolidated through column form, plain calculation, fill-in-the-blank items and word problems. All-addition and all-subtraction come first, then the mixed forms of add then subtract and subtract then add, before Addition Strategies, Subtraction Strategies and Addition and Subtraction Strategies pull the ideas together.

For 20 - 8 + 2, adding the last two numbers first to get 20 - 10 and answering 10 stands out. A regrouping trick learned in the strategy work has been carried into an expression that mixes subtraction and addition. Where two amounts are taken away one after the other, children also answer using only one of them.

The count of numbers grows from two to three, and this unit adds strategies for combining them efficiently along with that growth. Working through an expression step by step was already learned by Mixed Addition and Subtraction. That awareness turns into a formal rule in Rules of Calculation in grade 4, once all four operations mix together.