Place Value Addition: Finding Number Pairs That Sum to 15

Q: Why can't I use 20 + something to make 15?

Because 20 is already bigger than 15! You'd need to subtract, not add. Place value addition means we're only adding tens and units together.

Q: What if I used 0 tens and 15 units instead?

That doesn't follow the place value rule! Units must be single digits (0-9). The number 15 itself contains 1 ten and 5 units, so we write it as $ 10 + 5 $.

Q: Can there be other correct answers besides 10 + 5?

No! Since we need exactly 15 and units must be single digits, only $ 10 + 5 $ works. Any other tens value either exceeds 15 or requires units larger than 9.

Q: How do I know which tens value to try first?

Start with the largest possible tens value that doesn't exceed your target. For 15, that's 10. If targeting 25, you'd start with 20, and so on.

Q: What does 'using tens and units' actually mean?

It means breaking numbers into place value parts: tens place (multiples of 10) and units place (single digits 0-9). We're showing how 15 = 1 ten + 5 units.

Place Value Decomposition with Two-Digit Numbers

Which addition exercise using tens and units results in the number 15?

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Step-by-step written solution

Follow each step carefully to understand the complete solution

Understand the problem

Which addition exercise using tens and units results in the number 15?

Step-by-step solution

To determine which addition exercise using tens and units results in the number 15, follow these steps:

Step 1: Understand the requirements. We are asked to use a combination of:
- Tens (a multiple of 10).
- Units (a single-digit number).
to sum to 15.
Step 2: Consider possible tens values. Starting from the smallest possible multiple of 10, which is 10, because 20 or more would exceed 15 when adding any units.
Step 3: Calculate the necessary unit. Subtract the chosen tens value from the total desired sum: $15 - 10 = 5$ So, the necessary units value is 5.
Step 4: Verify the calculation. Combining the tens and units adds up to: $10 + 5 = 15$
Step 5: Confirm from choices. Among the given choices, $10 + 5$ is option number 1, and it meets our criteria correctly.

Therefore, the correct addition exercise using tens and units is $10 + 5$ , resulting in the number 15.

Final Answer

$10+5$

Key Points to Remember

Essential concepts to master this topic

Rule: Use one ten (10) plus units to reach 15
Technique: Subtract tens from target: 15 - 10 = 5 units needed
Check: Verify tens + units equals target: 10 + 5 = 15 ✓

Common Mistakes

Avoid these frequent errors

Using multiple tens or exceeding the target sum
Don't use 20 or higher for tens when targeting 15 = impossible sum! Any tens value above 10 makes it impossible to reach exactly 15 with single-digit units. Always start with the largest possible tens value that doesn't exceed your target.

Practice Quiz

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What number do the blue squares below represent?

FAQ

Everything you need to know about this question

Why can't I use 20 + something to make 15?

Because 20 is already bigger than 15! You'd need to subtract, not add. Place value addition means we're only adding tens and units together.

What if I used 0 tens and 15 units instead?

That doesn't follow the place value rule! Units must be single digits (0-9). The number 15 itself contains 1 ten and 5 units, so we write it as $10 + 5$ .

Can there be other correct answers besides 10 + 5?

No! Since we need exactly 15 and units must be single digits, only $10 + 5$ works. Any other tens value either exceeds 15 or requires units larger than 9.

How do I know which tens value to try first?

Start with the largest possible tens value that doesn't exceed your target. For 15, that's 10. If targeting 25, you'd start with 20, and so on.

What does 'using tens and units' actually mean?

It means breaking numbers into place value parts: tens place (multiples of 10) and units place (single digits 0-9). We're showing how 15 = 1 ten + 5 units.

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