Solve the Decimal Equation: Finding the Missing Number in 0.1 + ? = 0.4

Q: Why do I subtract 0.1 from 0.4 instead of adding them?

Think of it like this: if you have 0.4 total and already used 0.1, how much do you still need? You subtract to find the missing piece: $ 0.4 - 0.1 = 0.3 $.

Q: Can I solve this without rearranging the equation?

Yes! You can think: "What number plus 0.1 equals 0.4?" Count up from 0.1 to 0.4, and you'll get 0.3. Both methods give the same answer!

Q: How do I check if 0.3 is really correct?

Substitute 0.3 back into the original equation: $ 0.1 + 0.3 = 0.4 $. Since both sides equal 0.4, your answer is correct!

Q: What if I'm not good with decimal subtraction?

Try thinking in money terms! 0.4 is like 40 cents, and 0.1 is like 10 cents. 40¢ - 10¢ = 30¢, which is 0.3. This makes decimal subtraction easier to visualize.

Q: Is there a pattern I can use for similar problems?

Absolutely! For any equation $ a + ? = b $, the missing number is always $ ? = b - a $. Sum minus known addend equals missing addend!

Decimal Addition with Missing Addends

Fill in the missing values (?):

$0.1+?=0.4$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:07 Let's find the correct answer from the options.

00:11 We'll test each one by placing and checking carefully.

00:18 Using long multiplication for accurate calculation.

00:24 Let's compute and see if it fits the question.

00:36 The first option doesn't work, so let's try the second one.

00:48 We'll apply the same method and examine it closely.

01:07 The second option isn't correct, let's look at the third.

01:14 Again, we'll use the same approach to verify.

01:35 The third option fits, let's check the fourth for certainty.

01:39 We'll use the same method to inspect the fourth option.

02:03 The fourth one doesn't fit either.

02:11 And that's how we find the correct answer!

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

Fill in the missing values (?):

$0.1+?=0.4$

Step-by-step solution

To solve this problem, we need to find what number, when added to $0.1$ , results in $0.4$ . We can solve this using simple arithmetic operations as follows:

Step 1: Understand that the equation provided is $0.1 + ? = 0.4$ .
Step 2: To find the missing value $?$ , we need to rearrange the equation by isolating $?$ on one side: $? = 0.4 - 0.1$ .
Step 3: Conduct the subtraction: $0.4 - 0.1 = 0.3$ .

Therefore, the number that needs to be added to $0.1$ to equal $0.4$ is $0.3$ .

Reviewing the provided multiple-choice options, the correct choice is the third option: $0.3$ .

Final Answer

0.3

Key Points to Remember

Essential concepts to master this topic

Rule: To find missing addend, subtract known addend from sum
Technique: Rearrange equation: $0.1 + ? = 0.4$ becomes $? = 0.4 - 0.1$
Check: Substitute back: $0.1 + 0.3 = 0.4$ confirms our answer ✓

Common Mistakes

Avoid these frequent errors

Adding instead of subtracting to find the missing number
Don't add 0.1 + 0.4 = 0.5! This gives you the wrong operation and creates a larger number instead of finding what's missing. Always subtract the known addend from the sum to isolate the unknown value.

Practice Quiz

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FAQ

Everything you need to know about this question

Why do I subtract 0.1 from 0.4 instead of adding them?

Think of it like this: if you have 0.4 total and already used 0.1, how much do you still need? You subtract to find the missing piece: $0.4 - 0.1 = 0.3$ .

Can I solve this without rearranging the equation?

Yes! You can think: "What number plus 0.1 equals 0.4?" Count up from 0.1 to 0.4, and you'll get 0.3. Both methods give the same answer!

How do I check if 0.3 is really correct?

Substitute 0.3 back into the original equation: $0.1 + 0.3 = 0.4$ . Since both sides equal 0.4, your answer is correct!

What if I'm not good with decimal subtraction?

Try thinking in money terms! 0.4 is like 40 cents, and 0.1 is like 10 cents. 40¢ - 10¢ = 30¢, which is 0.3. This makes decimal subtraction easier to visualize.

Is there a pattern I can use for similar problems?

Absolutely! For any equation $a + ? = b$ , the missing number is always $? = b - a$ . Sum minus known addend equals missing addend!

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