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
The lesson
You already know how to add and subtract numbers up to 100. Big numbers work the same way, they just have one more place: hundreds. Line up the ones, tens, and hundreds. Then add or subtract each place, starting with the ones.
Add 347 + 256. Start with the ones: 7 + 6 = 13. Write the 3 and carry 1 ten. Next the tens: 4 + 5 + 1 = 10. Write the 0 and carry 1 hundred. Last the hundreds: 3 + 2 + 1 = 6. The answer is 603.
Subtract 502 - 268. The ones need 2 - 8, but there are 0 tens to borrow from, so first trade 1 hundred for 10 tens. Now you have 4 hundreds, 9 tens, and 12 ones. Ones: 12 - 8 = 4. Tens: 9 - 6 = 3. Hundreds: 4 - 2 = 2. The answer is 234.
Drawing hundreds, tens, and ones as blocks shows you exactly what is happening: 10 ones trade for 1 ten, and 10 tens trade for 1 hundred. The written steps do the same trade, just with numbers instead of blocks.
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.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.