Finding the Unknown Number

Learning how to find the unknown number in addition and subtraction equations is a first grade, Common Core math skill: 1.OA.8. 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 comparing the number of objects within two groups to determine whether the groups are equal. They should also be able to compare two numbers (between 1 and 10) and decide if they are equal (K.CC.6-7).

Future Learnings

Understanding equations and addition and subtraction will help your students understand principles later on in first grade as well as in second grade. Your students will be able to think about inequalities, continuing to use their understanding of the equal sign (1.NBT.4). 

Later in the second grade, your students will be able to write equations that express equivalent groups and use even numbers, equal parts, skip counting, etc. (2.OA.3-4). They will also be able to write equations to solve word problems (2.OA.1).

Common Core Standard: 1.OA.8 - Determine the unknown whole number in an addition or subtraction equation relating three whole numbers

Students who understand this principle can:

  1. Find the unknown number when it is in various places in addition equations. 
  2. Find the unknown number when it is in various places in subtraction equations.
  3. Explain how the unknown number was found. 

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: Finding the Missing Balloons

This video begins by explaining what a missing part in an equation is. It uses a “part-part-whole” chart, which has the total on top and the two numbers which add to equal that number underneath. 

The chart shows the equation 4 + __ = 10. The video then gives a word problem that matches the problem: “I got 10 balloons at the fair. 4 of the balloons were green. The rest were red. How many were red?”

  1. 10 circles are drawn to represent balloons. 
  2. 4 green balloons are placed in four circles, and the rest are given red balloons.
  3. After counting the red balloons, the total is added into the equation.
    a. 4 + 6 = 10
  4. Another way to look at the problem is 10 - 4 = 6. 
    a. So 6 is the missing number in the “part-part-whole” chart.

The video then gives another word problem for your students to try on their own: “My little sister got 3 pink balloons at the fair. Then she got some more yellow balloons! Now she has 5 balloons in all. How many are yellow?”

  1. A blank “part-part-whole” chart is shown and your students can fill in the blanks.
    a. 5 goes on the top.
    b. 3 goes in one of the part slots.
  2. Students must find what the second part is. 
  3. All three methods to help solve the problem are shown.
    a. The circles with 3 pink balloons; 
    b. 3 + __ = 5
    c. 5 - 3 = ?
  4. The answer is 2. 

All the methods are reviewed before the video ends.

Video 2: Learning how to Find Unknown Numbers

The video reviews that addition and subtraction are related. It uses the equation 7 + 5 = 12, stating that 7, 5, and 12 are part of a fact family. You can create another addition statement and two subtraction statements using these numbers: 5 + 7 = 12; 12 - 7 = 5; and 12 - 5 = 7.

The video explains that by understanding how fact families work, finding an unknown number is possible. A few practice examples are shown to help your students find the unknown number. 

  1. The first equation is 3 + __ = 10.
    a. Subtracting 3 from 10 can identify the missing number.
    b. 10 - 3 = 7
    c. The unknown number is 7, and you can check this by adding 3 to 7.
  2. The second equation is __ + 5 = 6.
    a. Subtract 5 from 6. 
    b. 6 - 5 = 1.
    c. The unknown number is 1.
  3. The third equation is 14 - __ = 10.
    a. Boddle explains that if the unknown number is one being subtracted (subtrahend), then “subtract the difference (10) from the minuend (14).”
    b. 14 - 10 = 4
    d. The unknown number is 4.
  4. The fourth equation is 16 - __ = 9.
    a. Subtract 9 from 16.
    b. 16 - 9 = 7.
    c. Check if it is correct by subtracting 7 from 16.
  5. The last equation is __ - 5 = 8.
    a. Boddle explains that if the minuend (the number before the minus sign) is the unknown number, then add the two known numbers. 
    b. So, 5 + 8 = 13.
    c. The unknown number is 13, and you can cross check the number by subtracting 5 from 13.

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.