March math review for 3rd grade, 4th grade and 5th grade on divisibility rules and the multiples of 3 and 9: CSI Mystery Detective worksheets that turn a hundreds chart into a crime scene where the suspect is a number under 100. The class runs a proper investigation, evidence to forensics to line-up to interrogation, and every stage takes more numbers off the board. The clue they keep for good: digits that add to 9 mean a multiple of 9. Nothing to cut, only pages to print. Students decode a cipher, shade the multiples, apply the digit-sum rule and finish on a color-the-clues puzzle that closes the mystery.
The Case: The Suspect Number
All that is known at the start is that the number is under 100, which is not much to go on. Your students run it as a real investigation - evidence, forensics, line-up, interrogation - and every stage takes more numbers off the board. The clue that does most of the work is the one your class will keep for good: add the digits, and if they come to 9, you have a multiple of 9.
What's Inside
- A letter-and-number cipher that decodes the opening message
- Shading the multiples of 3 and 9 on a hundreds chart
- Working out that every multiple of 9 is also a multiple of 3
- Using the digit-sum rule to narrow the suspect list further
- A final color-the-clues puzzle that separates the last two suspects
- Full color and black-and-white versions of every page
- Answer keys for the whole case
What Is a CSI Mystery Detective Case?
Take a math escape room, take away the locks and the app, and put a line-up where they were: that is a CSI Mystery Detective case. Students solve, each right answer clears one candidate, and the last standing closes the case. Nothing is cut, prepared or installed. A practice page lets a class drift; detective activities do not, because one wrong answer leaves more than one possibility standing. It works as a full math activity, in math centers, or as math worksheets to hand a substitute.
When to Use This Resource
- Introducing or consolidating multiples and divisibility rules
- Hundreds chart work with a purpose behind it
- Math centers, early finishers and independent work
- Emergency sub plans that run themselves
- Back-to-school weeks and end of year review
Grade Levels
Built for 3rd grade, 4th grade and 5th grade. Third graders usually need the digit-sum rule demonstrated once; fourth and fifth graders work it out for themselves and run the rest independently. It also suits 6th grade students who never properly learned the divisibility rules.
Why Teachers Love CSI Mystery Detective Cases
- Zero prep - print and hand out
- Self-checking by design, because an early error shows up later
- A divisibility rule students remember afterward
- Works as a whole lesson, a center, or a filler for an awkward afternoon
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