Montana Math MT.2.OA.2

Math2nd GradeOperations and Algebraic Thinking

The Standard

Flexibly, accurately, and efficiently add and subtract within 20 using mental strategies.

2026 Montana Mathematics Content Standards

Teacher's field guide

What Students Need to Do: Students add and subtract numbers within 20 in their heads. They choose a useful strategy, such as making 10, using doubles, or using an addition fact to subtract.
What Mastery Looks Like: For 8 + 7, a student makes 10 and adds the remaining 5 to get 15. For 15 - 8, they use 8 + 7 = 15 to find 7 and explain their choice.
Common Misconceptions: Some students count 8 as the first step when counting on for 8 + 7 and get 14. Others subtract 5 from 15 to reach 10, then forget to subtract the remaining 2.
How to Assess It: On an exit ticket, ask students to solve 8 + 7 and 15 - 8 without written steps. Have them write or tell you how they found each answer.

Lesson moves

Give pairs two ten-frames and 15 counters; have them build 8 + 7, then remove 8 and explain what remains.
Ask, “How can you use 10 to solve 9 + 6?” Have partners compare two mental paths.
Play a partner match game with cards showing facts like 7 + 8 and 15 - 8 alongside answer cards; explain each match.
Tell students the class has 16 pencils and gives away 9; ask how many remain and which mental shortcut they used.

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