Addition and subtraction within 1000
Add and subtract within 1000 using concrete models, drawings, and strategies based on place value; understand composing and decomposing tens and hundreds
What a learner can do afterwards
- Add two three-digit numbers using base-ten blocks or drawings, composing a ten or hundred when necessary
- Subtract three-digit numbers, decomposing a ten or hundred when necessary
- Relate the concrete/drawn strategy to a written method and explain why it works
1 · Read
You already know how to add and subtract numbers up to 100. Big numbers work the same way, with one more place: hundreds. Line up the places and start with the ones. If a place adds up to 10 or more, trade 10 of them for 1 of the next place. If a top digit is too small to take away from, trade 1 from the next place for 10.
Add 358 + 267. Ones: 8 + 7 = 15, so write 5 and carry 1 ten. Tens: 5 + 6 + 1 = 12, so write 2 and carry 1 hundred. Hundreds: 3 + 2 + 1 = 6. The answer is 625.
Subtract 604 - 287. There are 0 tens, so first exchange 1 hundred for 10 tens. Then exchange 1 ten for 10 ones: 5 hundreds, 9 tens and 14 ones. Now 14 - 7 = 7, 9 - 8 = 1 and 5 - 2 = 3. The answer is 317. Exchanging is also called borrowing.
Drawing hundreds, tens, and ones as blocks shows the trades. Ten ones become one ten, and ten tens become one hundred. For example, 11 tens can trade 10 tens for 1 hundred, leaving 1 ten. Subtraction trades the other way: 1 hundred becomes 10 tens. The written method records the same trades with digits.
Add or subtract one place at a time, and trade 10 ones for a ten or 10 tens for a hundred whenever you need to.
Practice and reading text written for the os-taxonomy project. Videos by Khan Academy, embedded from YouTube.
2 · Watch
Take it off screen
Where it sits
Where this leads
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