Learning how to find unknown addends and missing numbers is a first grade, Common Core math skill: 1.OA.4. Below we show two videos that demonstrate this standard. Then, we provide a breakdown of the specific steps in the videos to help you teach your class.

**Prior Learnings**

Your students should be familiar with the Kindergarten skill of understanding the number pairs that equal 10 and knowing all decompositions (e.g. 5=4+1, 5=2+3) of numbers below 10. This skill builds a foundation for strategy development, the understanding of place values, and properties of operations (K.OA.3-4). Your students should also understand that ten 1’s plus more 1’s are considered “teens” (K.NBT.1).

**Future Learnings**

Understanding how to perform addition and subtraction within 20 will enable your students to perform similar skills up to 100, eventually extending those skills to work with larger numbers and solve two-step word problems (2.OA.1). They will also be able to apply this skill with problems in a variety of contexts involving length, picture graphs and bar graphs (2.NBT.5).

**Common Core Standard: 1.OA.4 - Understand subtraction as an unknown-addend problem**

Students who understand this principle can:

- Use pictures, objects, numbers, etc. to explain the relationship between addition and subtraction problems.
- Represent how addition and subtraction are related by using pictures, objects, numbers, etc.
- Rewrite subtraction equations as addition equations which have a missing number.

**2 Videos to Help You Teach Common Core Standard: List Standard**

Below we provide and breakdown two videos to help you teach your students this standard.

**Video 1: Find the Missing Number**

The video begins by providing 2 example equations and asks if they have the same answer:

- 10 - 2 = ?
- 10 = 2 + ?

To help solve the problem, the video provides “counters” which represent the numbers. The first set of counters has 10, and 2 are taken away, ending with 8 counters.

The bottom set of counters also has 10, and represents the total in the second equation. The middle set has 2 counters, and the video asks how many counters must be added to the 2 in order to equal 10. The answer is 8.

Both equations have the same answer:

- 10 - 2 =
**8** - 10 = 2 +
**8**

This video demonstrates how to find missing addends in addition and subtraction problems by showing a simple method to count and solve the equation.

** Video 2: Find the Missing Number: Drinks and Equations**

The video begins by reviewing how fact families work and shows an example equation of a fact family.

“A fact family is a set of 3 numbers that can form 2 addition and 2 subtraction equations.”

- In the equation, 3 - 1 = 2,
- One can swap 1 and 2: (3 - 2 = 1) and still have the correct answer.
- The equation can also form two addition statements: (1 + 2 = 3; and 2 + 1 = 3).
- The numbers 1, 2, and 3 are a fact family.

Boddle then provides practice problems for your students to review what they have learned. The first practice problem uses refreshing, fruit drinks.

- There are a total of 10 drinks.
- 7 of those drinks are cherry flavored.
- How many are orange flavored?
- By subtracting 7 from 10, you can find the answer.

a. 10 - 7 = 3 - There are 3 orange flavored drinks.
- You can solve the problem if the number of orange drinks are known along with the total.

a. 10 - 3 = 7.

The video then provides additional practice problems for finding the unknown addend.

- 2 + __ = 7

a. You can subtract 2 from 7 to find the answer.

b. 2 + 5 = 7 - 3 + __ = 4

a. You can subtract 3 from 4 to find the answer.

b. 3 + 1 = 4 - 7 + __ = 9

a. You can subtract 7 from 9 to find the answer.

b. 7 + 2 = 9

**Want more practice?**

Give your students additional standards-aligned practice with Boddle Learning. Boddle includes questions related to Comparing and Measuring Lengths plus rewarding coins and games for your students to keep them engaged. Click here to sign up for Boddle Learning and create your first assignment today.

*Information on standards is gathered from The New Mexico Public Education Department's New Mexico Instructional Scope for Mathematics and the Common Core website.