Mystery Numbers: Find the Unknown! frames missing-number work as detective cases under a magnifying-glass illustration. Students solve six addition mysteries like 6 + ______ = 10, six subtraction mysteries like 12 - ______ = 8, then work Maya's seashell story: 7 shells, some more from her brother, 13 in all, with a box for drawing counters to check the answer.
1.OA.1
“Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.”
The seashell problem is an add-to story with the change unknown, and the sheet supplies the drawing space the standard's 'objects, drawings' wording expects.
1.OA.8
“Determine the unknown whole number in an addition or subtraction equation relating three whole numbers.”
Twelve mystery equations move the unknown through start, change and result positions across both operations.
The tip defines the equal sign as both sides having the same total and names the tools: counting on, counting back or a drawing.
Six equations with one number hidden, beginning 6 + ______ = 10 and ______ + 4 = 11.
Six more built on subtraction, including 12 - ______ = 8 and ______ - 3 = 6.
Maya collected 7 seashells and her brother gave her more; now she has 13. Students complete 7 + ______ = 13, answer in words, then draw a picture or counters in the checking box.
Whole-class detective framing
Read the tip aloud in your best detective voice, then let the framing carry the motivation; the math itself is straight missing-number work.
Counters at the ready
Keep counters on tables for Part 3. The checking box only earns its place if students actually verify 7 + 6 with objects.
Split by operation
Set Part 1 on day one and Part 2 the next day if your class still mixes up which way to count for subtraction.
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