Addition & Subtraction CSI Mystery Worksheets | Grades 3–6 | The Secret Number

October CSI math worksheets for addition and subtraction review, even and odd numbers, and multiples on a hundreds chart, run as a CSI Mystery Detective case for 3rd grade, 4th grade and 5th grade. It covers the ground a class needs before a test or at the end of a unit, as a number investigation rather than a drill. Each correct answer eliminates a suspect, and the last name standing is the culprit, so the mystery only closes when every sum is right. Students regroup, sort even from odd up to 100, shade multiples of 3, and work a clue backwards to check it holds. Answer keys and black-and-white pages included.

The Case

A number is hiding, and every clue narrows down what it can be. Your students shade a hundreds chart, sort the odds from the evens, and work each answer against the evidence until only one number survives.

What's Inside

• Addition and subtraction with regrouping
• Telling even numbers from odd numbers up to 100
• Finding multiples of 3 on a hundreds chart and shading the pattern
• Working a clue backwards from the answer to check it holds
• Reading a chart and matching your answers against it
• Answer keys included
• Color and black-and-white versions, so it prints clean on any copier

What Is a CSI Mystery Detective Case?

A CSI Mystery Detective case is math practice with a culprit at the end of it. The class works the page, each correct answer rules one out, and the case closes on the last one left. It runs like a math escape room with the hardware removed - no locks, no app, no prep. It is far harder to coast through than ordinary math worksheets, because the arithmetic has to hold or nothing lines up. Use it as a full math activity, in math centers, or as detective activities for early finishers.

When to Use This Resource

• Math review before a test or at the end of a unit
• Math centers and small groups
• Sub plans and emergency cover
• Early finishers and morning work
• Homework that does not need explaining twice
• The last lesson of the week, when nothing else lands

Grade Levels

Designed for 3rd grade, 4th grade and 5th grade. A confident 2nd grader can handle the even and odd pages, and a 5th grader who still trips on regrouping gets honest practice rather than busywork.

Why Teachers Love CSI Mystery Detective Cases

• No prep at all - print it and hand it out
• Self-checking, so you are not hunting through thirty pages for one slipped digit
• Every clue uses a different puzzle shape, so the pack holds attention to the last page
• Students go back and check their own working, because they want to name the answer
• Quiet, focused engagement without a single instruction repeated

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