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

A secret two-digit number has slipped away from the crime scene, and your students are the only detectives sharp enough to track it down. This printable Addition & Subtraction CSI Math Mystery turns add-and-subtract review into a full investigation, and it's built for Grades 3–6 classroom teachers, homeschool parents, and tutors who want focused math practice wrapped in a story students actually care about solving.

Students work through a series of clue pages, use their answers to eliminate suspects, and race to crack the case — all while getting meaningful practice with addition and subtraction within 100. No apps, no prep, no fuss.

Quick Details

Grade Levels: Grades 3–6 (best for 3rd–5th; works as a 6th-grade warm-up or review)
Format: Printable PDF
Resource Type: CSI Math Mystery worksheets
Math Skills: Addition & subtraction within 100, even and odd numbers, multiples of 3, comparing and sorting numbers
Pages: 9+ pages including answer key
Best For: Math centers, sub plans, early finishers, review, small groups, and homeschool math
Tech Needed: None — no apps, locks, passwords, or student logins required

Case Summary

A number is hiding somewhere between 1 and 100, and it isn't coming quietly. Your students pull on their detective coats, sharpen their pencils, and step up to the crime scene — the only way to identify the culprit is to follow the math.

The first clue is a coded message. Students crack the cipher one addition fact at a time, and the decoded note hands them their opening lead. From there the evidence stacks up: the suspect is a multiple of 3, it leaves a clear trail across the hundreds chart, and it hides among the even numbers. Each new fact quietly knocks more suspects off the board.

Then it's time to question the witnesses. A full page of addition and subtraction problems gives students the range they need to corner the number — but two suspects still refuse to crack. In the final interrogation, a hundreds-chart color puzzle forces the truth into the open and reveals exactly who did it.

Hand your students the evidence, point them at the suspects, and let them prove they can solve the case. The math is the only thing standing between them and the truth.

Skills Covered

Students will practice:

• Adding and subtracting one- and two-digit numbers within 100
• Single-digit addition with regrouping (code breaking)
• Identifying even and odd numbers to 100
• Identifying multiples of 3 and skip counting by 3
• Comparing and sorting numbers
• Completing a hundreds chart
• Using answers to eliminate suspects
• Logic, reasoning, and problem solving

What's Included

• CSI-style math mystery activity
• Addition and subtraction clue pages
• Code-breaker and hundreds-chart puzzles
• Suspect elimination activity
• Answer key
• Print-and-go PDF format

Best Used For

This printable CSI Math Mystery works well for:

• Back-to-school math review (and all-year use)
• Math centers and early finishers
• Substitute teacher plans
• Small groups and independent work
• Homeschool math
• End-of-unit review
• Addition and subtraction practice and reinforcement