Solve Decimal Addition: 0.7 + 0.8 Using Base-10 Blocks

Q: Why do I get 15 when I add 7 + 8 in the tenths place?

You're adding 7 tenths + 8 tenths = 15 tenths! Since 10 tenths make 1 whole, you have 1 whole and 5 tenths, which equals 1.5.

Q: How do the base-10 blocks help me understand this?

Each small square represents one tenth. When you combine 7 squares + 8 squares = 15 squares, you can group 10 squares into 1 whole unit, leaving 5 tenths extra!

Q: What if I don't regroup and just write 0.15?

That would be wrong! $ 0.15 $ means 1 tenth and 5 hundredths, which is much smaller than our answer of 1.5 (1 whole and 5 tenths).

Q: Is there an easier way to think about this problem?

Yes! Think of it as money: 70 cents + 80 cents = 150 cents = $1.50. The decimal works the same way!

Q: How can I check my answer without blocks?

Convert to fractions: $ \frac{7}{10} + \frac{8}{10} = \frac{15}{10} = \frac{3}{2} = 1.5 $. Or think: 0.7 is close to 0.8, so together they're about 1.5.

Decimal Addition with Regrouping

$0.7+0.8=$

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations

00:06 Let's solve this problem together.

00:09 We'll use long addition, step by step.

00:13 First, place the decimal point in the right spot.

00:17 Next, add the smallest digits first. Take your time.

00:24 Remember to carry the 1 when needed, and place the units correctly.

00:31 Add one to this part of our calculation, and place it carefully.

00:40 And there you have it! That's the solution!

Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

$0.7+0.8=$

Step-by-step solution

To solve the problem, let's add the decimals 0.7 and 0.8:

Step 1: Align the numbers by their decimal points:

$0.7$
$+0.8$

Step 2: Add the digits from right to left:

Tenths place: $7 + 8 = 15$

The result of $7 + 8$ gives $15$ . Since we are in the tenths column, the value is 1.5 (carrying the 1 over to the units place). Thus, the sum is $1.5$ .

Therefore, the solution to the problem is $\boxed{1.5}$ .

Final Answer

1.5

Key Points to Remember

Essential concepts to master this topic

Alignment: Line up decimal points vertically before adding digits
Technique: Add tenths column: 7 + 8 = 15 tenths = 1.5
Check: Verify using base-10 blocks: 7 tenths + 8 tenths = 15 tenths ✓

Common Mistakes

Avoid these frequent errors

Adding without considering place value regrouping
Don't just write 7 + 8 = 15 and stop there = missing the decimal meaning! This ignores that 15 tenths equals 1 whole and 5 tenths. Always convert your sum to proper decimal form when you get more than 9 in any place value.

Practice Quiz

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FAQ

Everything you need to know about this question

Why do I get 15 when I add 7 + 8 in the tenths place?

You're adding 7 tenths + 8 tenths = 15 tenths! Since 10 tenths make 1 whole, you have 1 whole and 5 tenths, which equals 1.5.

How do the base-10 blocks help me understand this?

Each small square represents one tenth. When you combine 7 squares + 8 squares = 15 squares, you can group 10 squares into 1 whole unit, leaving 5 tenths extra!

What if I don't regroup and just write 0.15?

That would be wrong! $0.15$ means 1 tenth and 5 hundredths, which is much smaller than our answer of 1.5 (1 whole and 5 tenths).

Is there an easier way to think about this problem?

Yes! Think of it as money: 70 cents + 80 cents = 150 cents = $1.50. The decimal works the same way!

How can I check my answer without blocks?

Convert to fractions: $\frac{7}{10} + \frac{8}{10} = \frac{15}{10} = \frac{3}{2} = 1.5$ . Or think: 0.7 is close to 0.8, so together they're about 1.5.

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