Mountain Gap Jumpers: Three Strategies for Subtraction to 100
Shared by A STEM Educator
Students will fluently apply place value decomposition, open number line counting-on, and the constant difference property to solve two-digit subtraction problems within 100, and justify which strategy is most efficient for a given problem.
Lesson Overview
Students explore three conceptual subtraction strategies — Place Value Decomposition, Open Number Line Counting On, and Constant Difference — using a mountain-climbing elevation narrative. Tasks thin-slice from no-regrouping decomposition through crossing tens boundaries to applying Constant Difference to simplify hard problems.
Materials
- Vertical whiteboards or large chart paper
- Dry-erase markers (1 per group)
- Erasers or socks
- Random grouping cards
- Printed Open Number Line strips (optional)
- Connecting cubes or base-ten blocks (optional anchor)
Scaffolded Task Progression
Two climbers are on the same mountain trail. Use Place Value Decomposition to find the gap.
Break apart the subtrahend: ___, then ___ ___
Draw base-ten blocks on your whiteboard to prove your answer.
The trail crosses a rocky ravine. Find the elevation gap using Place Value Decomposition.
Break apart : subtract the tens, then subtract the ones. Watch out — you will cross a ten boundary!
Show every step on your whiteboard.
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