Mixing It Up

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SWBAT write an equation for both an addition and subtraction story problem

Big Idea

Students think flexibly as they work to write equations for both addition and subtraction problems. No more plus and minus mix-ups!

Setting Up the Learning

5 minutes

This lesson aligns really well to CCSS Mathematical Practice 2, "Reason abstractly and quantitatively." In the lesson, students have to decontextualize the numbers and actions of the problem into symbols. They also have to find the evidence necessary to justify their claim that they need a + or - symbol. 


We have been looking at how to write equations to match story problems. Today we are going to mix + and – problems and see if you can write a matching equation for each of them.


Mathematicans use symbols to match their stories. When we use symbols it helps us explain what happened only using numbers.

Objective: Today your thinking job is: How can I write an equation that matches a put together or take away problem?

Opening Discussion

15 minutes

Let’s read both story problems. Before we even look at how we can solve them, let’s decide if they need a + or a – sign. We can’t write an equation until we have figured out which symbol matches.

I see 10 frogs in the pond.
Then 4 hop away.
How many frogs are in the pond now?


I see 10 frogs in the pond.
Then 4 more frogs come.
How many frogs are in the pond now?


After reading the problems, ask guiding questions: These are the questions kids need to internalize to help them "Make sense of problems.." (CCSS MP1). Asking them whole group helps kids practice with them until they naturally ask them on their own.

  • What symbol will I use to represent what happened in this problem?

  • Why will I use that symbol?

  • What evidence did you have to prove that you needed to put together or take away?

  • Will I have more or less frogs at the end?


After figuring out the symbols, I’ll just label each problem with a + or a – symbol on the chart paper.

Student Work Time and Strategy Share

20 minutes

Student Work Time:

I'll have students work on these two problems independently. Students will need 10 minutes at least to solve both problems today. This is a big chunk of the lesson, so this is a great time to target students who seem to be struggling.

See attached document for student share problems!

Student Share Time:

After students work on their problems, I'll bring them back together and have them share their number sentences with a partner.  

We will share the problems whole group also. The focus of the strategy share is on the equation, but I will do a quick 3-4 minute discussion on how each student solved the problem.

Focus question: How do we show what happened in numbers? Why do we use that symbol? 


5 minutes

Students share their work with a partner, and spiral back to the initial objective by answering the question: How did you figure out what the matching equation is?