3
Adding three-digit numbers
Discover Mathematics P3: Term 1, Lessons 51–60
GoalAdd three-digit numbers with different strategies and check the answer with another strategy.
New words
sum · the answer to an additionstrategy · a way of working out an answercheck · to solve again in a different way and see if the answer is the same
Explanation
There are many ways to add three-digit numbers. You can break the numbers apart by place value: for 345 + 278, add 300 + 200 = 500, 40 + 70 = 110 and 5 + 8 = 13, and then 500 + 110 + 13 = 623. You can make a friendly number: for 497 + 256, 497 is 3 less than 500, so 500 + 256 = 756, and then take away the 3 to get 753. You can count on: for 683 + 159, add 100 to get 783, add 50 to get 833 and add 9 to get 842. Choose the strategy that works for you. Then check your answer with a different strategy.
| Place | Add | Result |
|---|---|---|
| Hundreds | 300 + 200 | 500 |
| Tens | 40 + 70 | 110 |
| Ones | 5 + 8 | 13 |
| Total | 500 + 110 + 13 | 623 |
Worked examples
| Step | Work |
|---|---|
| Make a friendly number | 497 + 3 = 500 |
| Add | 500 + 256 = 756 |
| Give back the 3 | 756 − 3 = 753 |
Count on: 683 + 159. Jump 100 to 783, jump 50 to 833 and jump 9 to 842. So 683 + 159 = 842.
| Step | Work |
|---|---|
| Add hundreds | 345 + 200 = 545 |
| Add tens | 545 + 70 = 615 |
| Add ones | 615 + 8 = 623 |
Class activity
Strategy swap. Write four addition problems with three-digit numbers, for example 236 + 148. Solve each one with a strategy you choose, and then check two of them with a different strategy. Show your partner both strategies. Do the answers match?
Practice
1
Find 236 + 148.
200 + 100 = 300, 30 + 40 = 70 and 6 + 8 = 14. 300 + 70 + 14 = 384.
2
Find 475 + 326.
400 + 300 = 700, 70 + 20 = 90 and 5 + 6 = 11. 700 + 90 + 11 = 801.
3
Find 589 + 214 and check with a different strategy.
Count on: 589 + 200 = 789, then 789 + 14 = 803. Break apart: 500 + 200 = 700, 80 + 10 = 90, 9 + 4 = 13, and 700 + 90 + 13 = 803.
4
Use a friendly number to find 707 + 295.
295 is 5 less than 300. 707 + 300 = 1,007, and 1,007 − 5 = 1,002.
5
A bus carries 258 passengers in the morning and 367 in the afternoon. How many passengers is that in all?
258 + 367: 200 + 300 = 500, 50 + 60 = 110 and 8 + 7 = 15. 500 + 110 + 15 = 625 passengers. Check by counting on: 258 + 300 = 558, 558 + 60 = 618, 618 + 7 = 625.