Finding the Unknown Number (in Addition)
Basics on the topic Finding the Unknown Number (in Addition)
How to Find an Unknown Number
How do you do addition with missing numbers? Sometimes when adding, there is information missing that is needed in order to solve the problem. We also call this adding with an unknown number. In this video, we learn how to find the missing addend or unknown number.
Usually, we add one part plus another part to find the whole. We also sometimes call the parts addends, and the whole, sum. But, what happens when you don't know one of the parts? How can you find the missing number in addition? When you don’t know one of the parts, it is missing. How can we find missing addends?
Missing Number Addition – TwoDigit Numbers
Even if we are missing some parts and don’t know them, we can still use the parts, the whole, and addition to help us. In order to find the missing number, you can follow these steps:
 Start by setting up the addition equation using the plus sign
 Next, write in the numbers you know and put a question mark for the one you don't know.
 Then, count up from part one until you reach the whole. The amount of numbers we counted is the missing part!
Missing Number Addition – Examples
Let’s practice missing numbers in equations with the example below.
Mr. Squeaks has found one of his favorite snacks, carrots! He started with seven carrots and found some more. If he has sixteen carrots now, how many more carrots did he find? We can use missing number addition to help!
Step #  What to do 

1  Start by setting up the addition equation using the plus sign. 
2  Next, write in the numbers you know and put a question mark for the one you don't know. 
3  Then, count up from part one until you reach the whole. The amount of numbers we counted is the missing part! 
Following these steps, we need to count up from seven to sixteen. So, we can see that our missing number for the equation is nine!
Finding the Unknown Number in Addition – Summary
Today we learned how to find the missing number in addition. This can be helpful when you know some parts and a whole but are missing some information in an addition equation. Remember: These are the terms for addition in math:
Term  Meaning 

addends  The numbers that are being added 
sum  The result of adding two or more numbers 
Keep these steps in mind when looking for a missing number in addition:
 Start by setting up an addition equation
 Next, write in the numbers you know and put a question mark for the one you don't.
 Then, count up from part one until you reach the whole. That’s the missing number!
Want more practice on how to do missing number addition within the numbers 120? Take a look at the interactive exercises, worksheets, and activities on missing numbers in addition for 1st grade at the end of this video.
Transcript Finding the Unknown Number (in Addition)
: Mr. Squeaks and Imani are gathering food for winter hibernation, where they will stay in the cave for a long time. Mr. Squeaks found seeds first. He started with a pile of ten seeds, but now he has fourteen! How many seeds did Mr. Squeaks add? Let's figure it out by ... : "Finding the Unknown Number (in Addition)." Usually, we add one part plus another part to find the whole. We also sometimes call the parts addends, and the whole, sum. But, what happens when you don't know one of the parts? We can STILL use the parts, the whole, and addition to help us. First, set up the addition equation using the addition sign. Next, fill in the numbers we KNOW. Mr. Squeaks started with ten seeds, so write ten for the first part. He ended with fourteen seeds, so write fourteen for the whole. We can put a question mark in the space of the number we don't know. Then, count up from part one until you reach the whole. So, we will count from ten to fourteen. Count with me! Ten (...) eleven (...) twelve (...) thirteen (...) FOURTEEN(...) The amount of numbers we counted is the missing part! How many numbers are between ten and fourteen? Four. That means that ten plus FOUR equals fourteen... ... and Mr. Squeaks added four more seeds! Next, Mr. Squeaks added to his mushroom pile! If Mr. Squeaks started with eight mushrooms and ended with thirteen, how many did he add? Start by writing an equation. Where do we put the number of mushrooms Mr. Squeaks started with? We put eight, in the first part. We don't know how many Mr. Squeaks added yet, so we can put in a question mark to represent the missing part. We know the whole is thirteen, so we put that after the equal sign for the sum. Next count up from eight to thirteen. Count with me! eight (...) nine (...) ten (...) eleven (...) twelve (...) THIRTEEN. How many numbers did we count? We counted up five, so Mr. Squeaks added five mushrooms to his pile! Finally Mr. Squeaks found carrots for his pile. Mr. Squeaks started with seven carrots when he was gathering them, but now he has sixteen! How many carrots did Mr. Squeaks add to his pile? Remember, first write the equation. Where do the numbers go? We put seven in the first part and sixteen in the whole. Then, count up from seven to sixteen. Try it on your own! How many numbers did you count? When counting from seven to sixteen, we count NINE numbers. Let's review! Today we learned how to find the missing number in addition. First, write an equation Next, write in the numbers you know and put a question mark for the one you don't. Then, count up from part one until you reach the whole. Now, I wonder how hibernation prep is going. It looks like Mr. Squeaks AND Imani have had a lot to eat!
Finding the Unknown Number (in Addition) exercise

Where do the parts and wholes belong?
HintsRemember, the parts together make the whole.
SolutionAn equation is made of parts that equal a whole. We add all the parts together, which make the whole.

What is the unknown number?
HintsRemember, start at the part known and count up until you reach the whole.
Next, figure out how many numbers you counted up. That is the missing part.
SolutionThe unknown number is 7.
We start at 5, and count up until 12.
6, 7, 8, 9, 10, 11, 12. That's 7 numbers that we counted up.

Fill in the equation and find the missing number.
HintsRemember, the whole is the sum of the parts.
Remember, start at the part known and count up until you reach the whole.
Next, figure out how many numbers you counted up. That is the missing part.
SolutionThe parts (4 + 8 or 8 + 4) are added to equal the whole (12). The missing number is 4, since 8 + 4 = 12 or 4 + 8 = 12.

Match the unknown number.
HintsRemember, the two parts are on the top, and the whole is on the bottom, like this example.
Count on from one part until you reach the whole to find the unknown number.
SolutionWe found the matching pairs by understanding that part + part = whole.

What's the correct equation to find the unknown number?
HintsRemember, start with the numbers you know. Write an equation on your own using the part and whole provided.
Think about where the question mark should go in the equation. Is it a part of the equation, or the whole?
SolutionThe correct equation is 6 + ? = 12. We know one part is 6, and the whole is 12, so we add a question mark where the unknown number is.
If we were to solve for the ?, we would count up from 6 until 12. 6, 7, 8, 9, 10, 11, 12. We count up 6 numbers, so the unknown number is 6! 6 + 6 = 12.

Select the unknown number.
HintsRemember, find the missing number by counting up from the part to the whole.
Look at this number sentence: 7 + ___ = 14. To find the missing part, we count up from 7 until we reach 14. 7, 8, 9, 10, 11, 12, 13, 14. How many numbers did we count on? That is our missing part.
SolutionWe find the missing number by counting up from one part until we get to the whole. For example, in the number sentence 9 + ___ = 14, we start from 9, and count up until we reach 14. 9, 10, 11, 12, 13, 14. We counted up 5 numbers, we we matched this number sentence to the 5!