Missing Numbers in Equations

Sometimes, in a math problem, a number goes hiding! You might see a blank line ($\-\_$) or a question mark ($?$) instead of a number. Your job as a math detective is to find that missing number.

What is a Missing Number?

An equation is like a balanced scale. The amount on the left side of the equals sign ($=$) must be exactly the same as the amount on the right side.

If we have $? + 3 = 10$, we need to figure out what number goes in the place of the $?$ to make the whole thing equal $10$.

Finding Missing Numbers in Addition

Let's look at this problem: $\_\_ + 5 = 12$

This means, "What number plus $5$ gives us $12$?" There are two easy ways to solve this:

1. Count Up: Start at $5$ and count up until you reach $12$. ($6, 7, 8, 9, 10, 11, 12$). You counted $7$ times!
2. Use Subtraction: Addition and subtraction are best friends. If you know the total is $12$, you can subtract the part you know ($5$) to find the missing part. $12 - 5 = 7$

So, the missing number is $7$. Let's check: $7 + 5 = 12$. It works!

Finding Missing Numbers in Subtraction

Now let's try a subtraction problem: $15 - \_\_ = 8$

This asks, "If I start with $15$, how many do I take away to get $8$?" Again, we can use the relationship between addition and subtraction:

• You can think: "What number plus $8$ equals $15$?" ($8 + \_\_ = 15$)
• Or, you can just subtract the answer from the starting number: $15 - 8 = 7$

The missing number is $7$. Let's check: $15 - 7 = 8$. Perfect!

Let's Practice!

Try solving this one on your own: $? + 3 = 10$ Hint: What is $10 - 3$?

The missing number is $7$!

Remember, whether you are adding or subtracting, you can always use the numbers you do know to find the number that is hiding!