Addition Within 1000: Mental and Written Methods
What You'll Learn: How to add three-digit numbers using your mind and paper!
Part 1: Mental Addition with Round Numbers
Sometimes you can add numbers in your head without writing anything down! This works best with "round" numbers like 100, 200, or 300.
Adding Hundreds
When you add hundreds, just change the hundreds place and keep everything else the same!
Example: 245 + 100 = ?
Think: "I have 2 hundreds, and I'm adding 1 more hundred, so now I have 3 hundreds!"
245 + 100 = 345
Think: "I have 2 hundreds, and I'm adding 1 more hundred, so now I have 3 hundreds!"
245 + 100 = 345
Adding Tens
When adding tens, only the tens place changes!
Example: 234 + 30 = ?
Think: "I have 3 tens, and I'm adding 3 more tens, so now I have 6 tens!"
234 + 30 = 264
Think: "I have 3 tens, and I'm adding 3 more tens, so now I have 6 tens!"
234 + 30 = 264
Adding Ones
When adding ones, only the ones place changes!
Example: 325 + 4 = ?
Think: "I have 5 ones, and I'm adding 4 more ones, so now I have 9 ones!"
325 + 4 = 329
Think: "I have 5 ones, and I'm adding 4 more ones, so now I have 9 ones!"
325 + 4 = 329
Part 2: Addition Without Regrouping (No Carrying)
"Without regrouping" means each place value stays under 10, so we don't need to carry!
How It Works
Add each column separately: ones + ones, tens + tens, hundreds + hundreds.
Example: 234 + 152 = ?
234 + 152 ----- 386
Step by step:
• Ones: 4 + 2 = 6
• Tens: 3 + 5 = 8
• Hundreds: 2 + 1 = 3