STEMEngine AI — Engineering Active Thinking
Community Lessons/2nd Grade Mathematics
2nd Grade Mathematics Lesson

Unmasking the Story: Solving Result, Change, and Start Unknown Word Problems

Shared by A STEM Educator

2nd GradeBuilding Thinking Classrooms (BTC)CCSS.MATH.CONTENT.2.OA.A.150 min
Learning Objective

Students will identify whether the unknown is the Total or a Part in a word problem, construct a Part-Part-Whole tape diagram to represent the structure, and write and solve a matching addition or subtraction equation for Result Unknown, Change Unknown, and Start Unknown problem types within 20.

Lesson Overview

Students move from simple Result Unknown problems to complex Start Unknown problems by building Part-Part-Whole tape diagrams at vertical whiteboards. The progression changes only one structural variable at a time, helping students see that the tape diagram reveals what is missing — not keywords.

Materials

  • Vertical whiteboards or large chart paper taped to walls
  • Dry-erase markers (one per group)
  • Erasers or paper towels
  • Unifix cubes or counters (optional manipulatives)
  • Tape diagram template cards (optional scaffold)
  • Random grouping cards or sticks
  • Exit slip papers (pre-printed with blank tape diagram outlines)
  • Homework 'Keep-It-Thinking' sheets

Scaffolded Task Progression

1Task 1
Floor — Result Unknown 🐿️ A squirrel hid 7 acorns under an oak tree. Later, he hid 6 MORE acorns under the same tree. Draw a tape diagram on your board: one big box on TOP for the Total, and two small boxes BELOW for the Parts. Write the number 7 in one Part box, the number 6 in the other Part box, and a question mark in the Total box. How many acorns are under the tree in all? Write a matching equation.
2Task 2
Developing — Change Unknown (Joining) 🐿️ The squirrel had 8 acorns in his nest. During the afternoon, he brought back SOME MORE acorns. Now he has 15 acorns total! Draw your tape diagram. Ask yourself: Is 15 the Total or a Part? Where does the question mark go? Write TWO equations that could help you solve it: one addition equation and one subtraction equation. What is the missing number?

Unlock the full lesson

See all 6 scaffolded tasks, the full answer key, warm-up, exit slip, and a print-ready version — free to start.

Create a free account

Already have an account? Sign in

Make it your own

Like this lesson but want to tweak it?

Make an editable copy and adjust the tasks, warm-up, and answer key to fit your class — no need to start from scratch.

Generate a new lesson