How many did I hide?: Missing Part Game

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SWBAT identify the missing part and write a matching missing addend equation.

Big Idea

Expect high investment AND rigor in this lesson! Students get to play an interactive game as they work on identifying the missing addend in a number sentence!

Setting Up the Learning

5 minutes

Missing addend is explicitly mentioned within the 1.OA.1 standard ("unknowns in all positions"). In this lesson, students start to apply what they know about missing parts to a situation that isn't written in a story problem, helping them start to understand just the number sentence. This lesson helps them "Reason abstractly and quantitatively" (MP2). Students can represent what they are acting out in an equation!


We have been solving story problems where we add an unknown amount or subtract an unknown amount. Today we are going to play a game where we continue working on this math concept.

Objective: While we play this game, your thinking job is: How can I figure out how many my partner is hiding? How can I write that down in a number sentence?

Opening Discussion

10 minutes

Today we are going to play a game called Counters in a Cup. The object of this game is to figure out how many your partner is hiding. Some of the counters are showing, some of them are hiding-you have to figure out the missing part and then write it in a number sentence.


  1. Take out 12 counters. 12 is how many you have in all. Every partner group must have 12.
  2. Partner 1 closes their eyes. Partner 2 hides some counters under the cup.
  3. Partner 1 opens their eyes and counts how many counters they can see.
  4. Partner 2 asks: How many am I hiding?
  5. Partner 1 figures out how many Partner 2 is hiding.
  6. Both partners figure out a matching number sentence.

I'll model playing one round for students, and have the students keep track of the game steps to help me. 

When I get to Step 5, or the point where I need to figure out how many are hiding, i'll ask:

What are some strategies you could use to figure out how many counters are hiding?

Scaffolding Questions:

  • What information do we know? There are 12 in all, and ____ are outside the cup.
  • What information are we trying to figure out? (how many counters are inside the cup/how many are hiding)
  • What can I do to figure out how many are inside the cup?

Possible strategies: Counting on (using cubes, the number line or fingers), counting back (using fingers), known facts

I'll record some of their strategies on chart paper, using mostly visuals. Students need the visual to help them later when they play on their own. Using too many words is tricky for first graders!

Game Practice!

15 minutes

We practice the game whole group to help students get comfortable with the steps before they go and play on their own! As we play, I'll record what we do on a chart paper recording sheet. See the Counter in a Cup recording sheet for what it looks like!

I'll label each student either Partner 1 or 2. 

Sample round:

  • Partner 2s, close your eyes!
  • I'll roll a dice and show partner 1 how many to hide
  • Partner 2, open your eyes! How many are outside the cup? (7)
  • Model writing number sentence: 7 + ___ = 12: "I have 7 outside the cup. If I get those out of the cup, I'll have 12 again"
  • Partner 2s, figure out what is missing. Talk to your partner about what you think is missing and how you knew.

I'll float to hear different strategies. I'll push partner groups to ask each other how they know.

I'll choose 1-2 students to share how they figured it out. We will switch roles and play a few rounds each until students are comfortable with the game.

Independent Practice

15 minutes

Students play the game with a partner. The most challenging part of this game is writing the number sentence and identifying the missing part. What I do is tell students we always put a box around the missing part. The missing part is what we are trying to figure out. 

Group A: Intervention Scaffold

Students get a grid of 12 as a scaffold. They can put the counters on the grid that are outside the cup, the remaining boxes help them "see" how many must be hiding. 

Group B: Right on Track

Students play the game as planned, recording the number sentences.

Group C: Extension

Students record an addition and subtraction fact with each round of the game. This helps them connect subtraction to a missing addend problem, which is a concept we will address with 1.OA.B.4, "Understand subtraction as an unknown-addend problem." Having them connect these two facts is a start towards them understanding how they are related. 

See attached recording sheet: Counter in a Cup.

You can watch two groups play this game in the attached videos: Game Time Video and Game Video 2! It will give you a good sense for how the game works and also what strategies you can anticipate!


5 minutes

As a push for the end of this lesson, I want students to critically think about how they could apply what they practiced in the game to a number sentence that is outside of the context of the game. This is especially aligned to MP2, Reason abstractly and quantitatively. Now that students have had the concrete experience of actually hiding counters in a cup, they can now imagine that scenario based on just numbers. 

I'll shows a recording sheet on chart paper. In the number sentence portion it will say:

7+ ___ =10

Guiding Questions:

  • What does the 10 tell us? What does the 7 tell us? 
  • What will go in the blank? How can we figure that out?