Halloween Addition & Subtraction Within 100 CSI Mystery Worksheets

Inside the case file

October Halloween CSI math worksheets on addition and subtraction within 100, built as a CSI Mystery Detective case about the house on the corner that everybody has a story about. Pitched at 2nd grade and 3rd grade, it gives the class a subtraction mystery with an actual monster at the end, and it prints in color or black and white with nothing to prepare. This week somebody actually went inside and came back out with a description that matches nothing anyone has heard before. Answer keys are included. Students check worked answers for errors, color the correct arrow and follow linked clues to the monster's final age, crossing off a suspect with each correct answer.

The Case: Who Haunts The House?

You do not really believe the stories your friends tell about demons and hauntings, but they are good stories, and the best of them is always about the house on the corner. Everyone knows the one. This week somebody actually went in, and came back out again with a description that does not match anything anybody has heard before.

What's Inside

• Checking worked answers for errors and tallying which are correct
• Addition within 100 across the clue pages
• Addition problems used to color the correct arrow to the next problem
• Two-digit subtraction used to settle a clue about the monster
• A sequence of linked clues that builds to the monster's final age
• Full color and black-and-white versions of every page
• Answer keys for the whole case

What Is a CSI Mystery Detective Case?

The premise of a CSI Mystery Detective case is that the math breaks the alibis. Students solve, each correct answer removes one suspect from the line-up, and the last name standing is the one who did it. It is a math escape room with no locks, no app and nothing to set up. Detective activities are harder to coast through than ordinary math worksheets, because the deduction only closes on correct work. It fills a whole math activity, suits math centers, and works as a sub plan.

When to Use This Resource

• Halloween week in October, when a normal worksheet will not hold them
• Addition and subtraction within 100 review before or after a unit test
• Math centers, early finishers and independent work
• Emergency sub plans that run themselves
• End of year review, when attention is thin and the work still has to happen

Grade Levels

Built for 2nd grade and 3rd grade. Second graders run it comfortably, which makes it a good one for a class that needs a win; third graders finish inside a lesson. It also suits 1st grade students who are ahead and ready for two-digit work with something at stake.

Why Teachers Love CSI Mystery Detective Cases

• Zero prep - print and hand out
• Self-checking by design, because a wrong answer breaks the deduction
• A Halloween activity that still does real math
• Works as a whole lesson, a center, or a filler for an awkward afternoon

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