Solve for the Missing Addend in 5.8 + ? = 9.9

Q: Why do I subtract 5.8 from 9.9 instead of adding?

Because subtraction is the inverse of addition! When you have 5.8 + ? = 9.9, you need to undo the addition by subtracting 5.8 from both sides.

Q: How do I line up the decimal points when subtracting?

Always align the decimal points vertically: 9.9 - 5.8 ----- 4.1This ensures you subtract tenths from tenths and ones from ones!

Q: What if I get a different decimal answer?

Double-check your decimal alignment and borrowing. Remember: 9.9 - 5.8 should give you 4.1, not 4.0 or 4.2!

Q: How do I know my answer is correct?

Always substitute your answer back into the original equation: $ 5.8 + 4.1 = 9.9 $. If both sides are equal, you're right!

Missing Addend Problems with Decimal Numbers

Fill in the missing value (?):

$5.8+?=9.9$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:06 Let's find the correct answer from these options.

00:09 Try each answer in the expression to see which one works.

00:17 Use vertical addition to perform the calculations.

00:22 Plug in an answer and calculate. Ready?

00:43 The first option doesn't work. Let's check the second one.

00:48 Use the same method and check again.

00:57 Try another answer and calculate it.

01:10 The second option works! Now, let's try the third.

01:16 Use the same method to check once more.

01:26 Try one of the remaining answers and solve it.

01:52 The third option isn't correct. Let's go to the fourth.

02:02 Apply the same method again.

02:10 Insert the last answer and calculate.

02:31 The fourth option also doesn't work.

02:36 And that's how we find the right solution!

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

Fill in the missing value (?):

$5.8+?=9.9$

Step-by-step solution

To solve this problem, we need to determine the missing value in the equation $5.8 + ? = 9.9$ .

First, let's isolate the unknown value by rearranging the equation. We know that $? = 9.9 - 5.8$ .
Next, perform the subtraction: $9.9 - 5.8$ .

Let's calculate:

Align the decimal places and subtract:

  9.9
- 5.8
-----
  4.1

The result of $9.9 - 5.8$ is $4.1$ .

Therefore, the missing value is $4.1$ . Hence, the equation becomes $5.8 + 4.1 = 9.9$ .

Therefore, the correct answer is option 2: 4.1.

Final Answer

4.1

Key Points to Remember

Essential concepts to master this topic

Inverse Operations: Use subtraction to find the missing addend value
Technique: Rearrange to ? = 9.9 - 5.8 = 4.1
Check: Substitute back: 5.8 + 4.1 = 9.9 ✓

Common Mistakes

Avoid these frequent errors

Adding instead of subtracting to isolate the unknown
Don't add 5.8 to both sides = 5.8 + ? + 5.8 = 9.9 + 5.8! This creates a more complex equation instead of isolating the unknown. Always use the inverse operation - subtract 5.8 from both sides.

Practice Quiz

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FAQ

Everything you need to know about this question

Why do I subtract 5.8 from 9.9 instead of adding?

Because subtraction is the inverse of addition! When you have 5.8 + ? = 9.9, you need to undo the addition by subtracting 5.8 from both sides.

How do I line up the decimal points when subtracting?

Always align the decimal points vertically:

  9.9
- 5.8
-----
  4.1

This ensures you subtract tenths from tenths and ones from ones!

What if I get a different decimal answer?

Double-check your decimal alignment and borrowing. Remember: 9.9 - 5.8 should give you 4.1, not 4.0 or 4.2!

Can I solve this problem using addition instead?

You could guess and check by trying each answer choice, but subtraction is much faster! Use $? = 9.9 - 5.8$ to solve directly.

How do I know my answer is correct?

Always substitute your answer back into the original equation: $5.8 + 4.1 = 9.9$ . If both sides are equal, you're right!

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