Montana Math MT.1.OA.9

Math1st GradeOperations and Algebraic Thinking

The Standard

Determine the unknown number in an addition or subtraction equation relating to three numbers.

2026 Montana Mathematics Content Standards

Teacher's field guide

What Students Need to Do: Students find the missing number in an addition or subtraction equation. The blank may come first, middle, or last. They use counters, drawings, or related facts to check their answer.
What Mastery Looks Like: A student solves 7 + ? = 12 and ? - 5 = 8 without assuming the blank comes last. They put each number back into the equation to check it.
Common Misconceptions: Students may add the two visible numbers in 7 + ? = 12 and write 19. In ? - 5 = 8, they may write 3 because they subtract 5 from 8 instead of finding the starting number.
How to Assess It: Give students 6 + ? = 14 and ? - 4 = 9. Ask them to fill each blank and draw counters or write a related fact to check it.

Lesson moves

Give pairs counters and 8 + ? = 13, then have them build 8 and add counters until they reach 13.
Ask students to explain with a drawing why 12 - ? = 7 needs 5 in the blank.
Give pairs cards labeled 4, 6, and 10; have them make two true equations, hide one card, and quiz a partner.
Show 7 apples left after 4 were eaten, then ask students to write ? - 4 = 7 and find the starting amount.

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