Adding 3 Numbers with Regrouping (2-Digit Addends)

Inside the case file

Adding three numbers worksheets for 2nd grade and 3rd grade: five Mystery Detective math case files of three addends in one column, 2 digit addition with regrouping. No prep math activities with full answer keys, print and go, for math centers, morning work, early finishers, homework and sub plans. Somebody is doing quiet good deeds, a phone went missing between classes and the teacher's lunch vanished before noon; the right total crosses a suspect off the lineup and every total matches one suspect only, so a CSI Mini Mystery Detective does its own checking.

Adding three numbers in a column is the step straight after adding two, and it is the step where a class needs a full week of practice rather than one page, which five CSI Mini Mystery Detective math cases give you. Students add, find the suspect with that total and cross the name off, so a slipped carry shows up on the lineup and they go back themselves.

The Cases

1. Showing Mindfulness - quiet good deeds around school
2. The Missing Phone - it disappeared between classes
3. The Teachers Lunch - the lunch disappears before noon
4. Finding Rubbish - somebody is quietly cleaning up
5. Gift for the Teacher - a present with no name on it

What's Inside

• Five case files, each with a crime sheet and its own lineup of suspects
• Adding three numbers in one column, with two digit addends and regrouping
• Totals that carry, so students have to add all three digits before moving on
• Each answer matches one suspect and no other, so the page checks itself
• A full answer key for every case
• Print and go: no cutting, no laminating, nothing to prepare

What Is a CSI Mini Mystery Detective?

A CSI Mini Mystery Detective is one page, one crime and one class period. The suspects stand in a lineup with a number each, the questions on the crime sheet produce those numbers, and a correct answer knocks a suspect out until the last name standing is the culprit. There is nothing to cut or set up, so the arithmetic carries the whole thing, which is what separates detective activities from ordinary math worksheets.

When to Use This Resource

Three addends is the moment addition stops being automatic, because students have to hold a running total instead of just adding a pair. Use this the week you introduce it, then again whenever you see the class adding the first two numbers and forgetting the third. A wrong total names the wrong suspect, so the missing addend is caught on the page and not in your grading.

It works for test prep, homework, math centers, morning work, early finishers, sub plans and homeschool lessons, with a class, a small group or one child alone.

Grade Levels

Written for 2nd grade and 3rd grade, where adding three two digit numbers sits in the standards. First graders take it late in the year with single digit addends, and fourth graders use it as a quick fluency check.

Why Teachers Love These Cases

• Built on holding a running total, which is the actual difficulty
• Students notice a forgotten addend when the wrong suspect comes off
• No prep, and the answer keys make grading quick
• Works with a whole class, a small group, partners, or one student alone

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