Solve the Equation: Finding the Missing Value in 1800 + □ = 2100

Q: Why do I subtract instead of add to find the missing number?

Think of it like this: if you have 1800 + something = 2100, you need to find what was added to 1800. Since addition and subtraction are opposite operations, you undo the addition by subtracting!

Q: How can I remember which number to subtract from which?

Always subtract the known addend (the number already given) from the sum (the total). In this case: sum - known addend = 2100 - 1800 = 300.

Q: What if I get confused about which number is which?

Label them! In $ 1800 + \Box = 2100 $, we have: Known addend: 1800Missing addend: □Sum: 2100 Then use: Missing addend = Sum - Known addend

Q: Can I check my answer a different way?

Yes! You can also think backwards: Does 1800 + 300 equal 2100? If yes, your answer is correct. This is the most reliable way to verify your solution.

Q: What if the numbers were different, like smaller ones?

The same method works! For example, if you had $ 15 + \Box = 23 $, you'd calculate 23 - 15 = 8. The process stays the same regardless of number size.

Missing Addend Problems with Large Numbers

Complete the following Equation:

$1800+\Box=2100$

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Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

Complete the following Equation:

$1800+\Box=2100$

Step-by-step solution

To solve the problem $1800 + \Box = 2100$ , we need to determine the value of $\Box$ that, when added to 1800, equals 2100.

We'll follow these steps:

Step 1: Rearrange the equation to solve for the unknown $\Box$ .
Step 2: Perform the subtraction: $\Box = 2100 - 1800$ .

Let's perform the calculation:

Step 1: Write the equation for $\Box$ :

$\Box = 2100 - 1800$

Step 2: Calculate the difference:

$2100 - 1800 = 300$

Thus, the value of $\Box$ is $300$ .

Therefore, the solution to the problem is $\Box = 300$ .

Final Answer

$300$

Key Points to Remember

Essential concepts to master this topic

Rule: Subtract the known addend from the sum to find the missing addend
Technique: Rearrange 1800 + □ = 2100 to □ = 2100 - 1800
Check: Verify by adding: 1800 + 300 = 2100 ✓

Common Mistakes

Avoid these frequent errors

Subtracting in the wrong direction
Don't calculate 1800 - 2100 = -300! This gives a negative answer because you're subtracting the larger number from the smaller one. Always subtract the known addend from the sum: 2100 - 1800 = 300.

Practice Quiz

Test your knowledge with interactive questions

Choose the correct answer:

$693+705=\text{ ?}$

FAQ

Everything you need to know about this question

Why do I subtract instead of add to find the missing number?

Think of it like this: if you have 1800 + something = 2100, you need to find what was added to 1800. Since addition and subtraction are opposite operations, you undo the addition by subtracting!

How can I remember which number to subtract from which?

Always subtract the known addend (the number already given) from the sum (the total). In this case: sum - known addend = 2100 - 1800 = 300.

What if I get confused about which number is which?

Label them! In $1800 + \Box = 2100$ , we have:

Known addend: 1800
Missing addend: □
Sum: 2100

Then use: Missing addend = Sum - Known addend

Can I check my answer a different way?

Yes! You can also think backwards: Does 1800 + 300 equal 2100? If yes, your answer is correct. This is the most reliable way to verify your solution.

What if the numbers were different, like smaller ones?

The same method works! For example, if you had $15 + \Box = 23$ , you'd calculate 23 - 15 = 8. The process stays the same regardless of number size.

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