Lesson 4.2 · Adding and Subtracting Within 1,000
Adding three-digit numbers
You already know how to add two-digit numbers. Three-digit numbers work the same way. You just have one more place: the hundreds.
Add each place
Find 324 + 152.
Split each number into hundreds, tens and ones. Then add the same places.
| hundreds | tens | ones | |
|---|---|---|---|
| 324 | 3 | 2 | 4 |
| 152 | 1 | 5 | 2 |
| sum | 4 | 7 | 6 |
Ones: 4 + 2 = 6. Tens: 2 + 5 = 7. Hundreds: 3 + 1 = 4.
So 324 + 152 = 476.
When a place makes 10 or more
Sometimes a place adds up to 10 or more. Then you regroup.
- 10 ones make 1 ten. Move it to the tens place.
- 10 tens make 1 hundred. Move it to the hundreds place.
Adding three-digit numbers
Line up the places. Add the ones, then the tens, then the hundreds.
If a place has 10 or more, regroup: trade 10 for 1 in the next place to the left.
Worked example: 258 + 137
Ones: 8 + 7 = 15. That is 1 ten and 5 ones. Write 5. Move the 1 ten over.
Tens: 5 + 3 + 1 = 9. Write 9.
Hundreds: 2 + 1 = 3. Write 3.
The answer is 395.
Worked example: 467 + 285
Ones: 7 + 5 = 12. Write 2. Move 1 ten over.
Tens: 6 + 8 + 1 = 15. That is 15 tens, or 1 hundred and 5 tens. Write 5. Move 1 hundred over.
Hundreds: 4 + 2 + 1 = 7. Write 7.
The answer is 752.
Tip
Check with a quick guess. 467 is close to 500 and 285 is close to 300. So the answer should be close to 800. 752 makes sense!
Common mistake
Don't forget the number you moved over. In 467 + 285, the tens are 6 + 8 + 1, not just 6 + 8.
Practice
What is 231 + 145?
Type a number.
What is 406 + 372?
Type a number.
What is 348 + 226?
Type a number.
What is 563 + 181?
Type a number.
What is 379 + 456?
Type a number.
What is 295 + 308?
The library has 486 books. It gets 247 new books. How many books does it have now?
Type a number.
What number is 3 hundreds + 14 tens + 5 ones?
Type a number.