Adding Three-Digit Numbers and Two-Digit Numbers

Adding Three-Digit Numbers and Two-Digit Numbers is a Grade 2 mental-math routine built around decomposition and recomposition. Students break a three-digit number into friendlier parts, combine the ones with the two-digit addend, and then recombine the result. The point is not one fixed trick but learning to reshape numbers so an addition problem becomes easier to hold and solve mentally.

Use it as a whole-class number talk or strategy warm-up when you want students comparing different ways to make the same calculation manageable. Invite multiple decompositions and discuss which one feels most efficient. Create a free account to generate teacher-ready lesson support for this activity. No credit card required; 3 lesson generations for life.