Adding and subtracting within 1000 (grade 2) · lesson

Add hundreds

💡 Adding hundreds is like adding small numbers, but in hundreds.

300 + 400 is like 3 + 4. So 7 hundreds, or 700.

600 + 200 + 50 is 8 hundreds and 5 tens. That is 850.

🧠 How to think about it

Say it in hundreds: "3 hundreds plus 4 hundreds is 7 hundreds."

🏡 Try it at home

Pretend each dollar is 100. Show 3 dollars and 4 dollars. How many hundreds?

💡 Adding or taking 10 or 100 changes just one digit, unless you cross a zero.

100 more than 450 is 550. 900 − 100 is 800.

10 less than 702 is 692. There are no tens, so trade a hundred.

🧠 How to think about it

Find the digit that changes. 10 changes the tens. 100 changes the hundreds.

🏡 Try it at home

Start at 250. Take turns saying 100 more. Then 10 more.

💡 Add ones, then tens, then hundreds, and carry when a place makes 10.

245 + 132. Ones: 7. Tens: 7. Hundreds: 3. So 377.

368 + 157. Carry once from the ones and once from the tens. 525.

🧠 How to think about it

Check each place. Did it make 10 or more? Write the ones digit and carry 1 to the next place.

🏡 Try it at home

Add two page numbers from a book, place by place, on paper.

💡 Subtract place by place, and trade from the next place if you need more.

586 − 243. Ones: 3. Tens: 4. Hundreds: 3. So 343.

653 − 218. You can't take 8 from 3, so trade a ten. 435.

🧠 How to think about it

Never take the small digit from the big one just because it's easier. Trade first.

🏡 Try it at home

Show 653 with play money in hundreds, tens, and ones. Take away 218. Trade when you need to.

💡 When a place is zero, trade from the next place over first.

402 − 157. There are no tens. Trade 1 hundred for 10 tens.

Now trade 1 ten for 10 ones. You have 3 hundreds, 9 tens, 12 ones. The answer is 245.

🧠 How to think about it

Another way: 500 − 198. Take 200, which is 2 too many. Then add 2 back. 302.

🏡 Try it at home

Have 4 dollars and pay 1 dollar and 57 cents in play money. Make change step by step.