Finding the Unknown Number (in Addition)
Do you want to learn faster and more easily?
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 – Two-Digit 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 the numbers you know into a part whole model 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 Problems – Examples
Let’s practise missing number addition within 20 with the example below.
Mr. Squeaks has found one of his favourite 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! We can now complete our part whole model.
Addition (Missing Number) – Summary
Today we learnt 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 maths:
| 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 the numbers you know into a part whole model 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 1-20? Take a look at our missing number addition worksheets and interactive exercises as well as activities on missing number addition KS1 at the end of this video.
Finding the Unknown Number (in Addition) exercise
-
Where do the parts and wholes belong?
HintsLook at the size of the boxes to give you a clue. What is bigger - a part or the whole?
Remember, the parts together make the whole.
SolutionAn equation is made of parts that equal a whole. We add all of the parts together to make the whole.
-
What is the unknown number?
HintsRemember, start at the part that is known and count up until you reach the whole.
How many numbers did you count? 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. We counted up 7 numbers.
-
How many screws does Imani have?
HintsRemember, the whole is the sum of the parts.
Start at the known part and count up until you reach the whole.
How many numbers did you count up? That is the missing part.
SolutionThe missing number is 4.
If we count on from 8 to 12, we count 4 numbers, therefore 4 is the missing part.
You could have completed the equation with:
- 4 + 8 = 12
- 8 + 4 = 12
-
Which number is missing?
HintsAre you looking for a part or a whole?
In these part whole models, the two parts are in the smaller boxes at the top and the whole is in the larger box at the bottom.
If you are looking for a part, you need to count on from the known part to the whole. If you are looking for the whole, you need to add the two parts together.
SolutionWe found the matching pairs by understanding that a part + a part = a whole.
-
What is 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?
For example, if we were writing an equation for this part whole model, we could write 2 + ? = 4.
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 this question, we would count up from 6 to 12.
6, 7, 8, 9, 10, 11, 12.
We count up 6 numbers, so the unknown number is 6. 6 + 6 = 12.
-
What is the unknown number?
HintsRemember, find the missing number by counting up from the part to the whole.
or
Add the two parts together to find the whole.
Look at this number sentence: 3 + ? = 10. To find the missing part, we count up from 3 until we reach 10.
3, 4, 5, 6, 7, 8, 9, 10.
How many numbers did we count on? That is our missing part.
SolutionHere we can see which equations had which missing number.
If we are finding a missing part, we count up from the known part to the whole.
- For example, 9 + ? = 14.
- We count 9, 10, 11, 12, 13, 14.
- We counted 5 numbers so the missing number is 5.
- 9 + 5 = 14.
- For example, 5 + 2 = ?.
- We can count on 2 more from 5.
- 5, 6, 7, therefore the whole is 7.
- 5 + 2 = 7.
