Adding and Subtracting up to 1,000
What this lesson covers
- A fact you know helps with bigger numbers: 7 + 2 = 9, so 700 + 200 = 900.
- Pairs that make 100 help with lots of sums.
- When a column reaches 10 or more, rename 10 ones as 1 ten, or 10 tens as 1 hundred.
- In column methods, ones go under ones, tens under tens and hundreds under hundreds.
Big sums from small facts
- Use a fact you already know.
- Tens and hundreds work the same way as ones.
| Fact you know | With tens | With hundreds |
|---|---|---|
| 7 + 2 = 9 | 70 + 20 = 90 | 700 + 200 = 900 |
| 8 − 3 = 5 | 80 − 30 = 50 | 800 − 300 = 500 |
| 6 + 4 = 10 | 60 + 40 = 100 | 600 + 400 = 1,000 |
Making 100
- 34 + __ = 100. Count on to the next ten: 34 + 6 = 40.
- Then 40 + 60 = 100.
- 6 + 60 = 66, so 34 + 66 = 100.
| Pair | Total |
|---|---|
| 34 + 66 | 100 |
| 71 + 29 | 100 |
| 55 + 45 | 100 |
| 18 + 82 | 100 |
Changing one place at a time
- Think about which place is changing.
| Sum | Think | Answer |
|---|---|---|
| 329 + 3 | 9 ones + 3 ones = 12 ones | 332 |
| 329 + 80 | 2 tens + 8 tens = 10 tens, which is 1 hundred | 409 |
| 329 − 200 | 3 hundreds − 2 hundreds = 1 hundred | 129 |
Adding in columns: 137 + 54
- Line up the ones, tens and hundreds.
- Ones: 7 + 4 = 11. Write 1, and rename 10 ones as 1 ten.
- Tens: 3 + 5 + 1 = 9.
- Hundreds: 1 + 0 = 1.
- 137 + 54 = 191.
| Hundreds | Tens | Ones | |
|---|---|---|---|
| 1 | 3 | 7 | |
| + | 5 | 4 | |
| = | 1 | 9 | 1 |
Try it: add in columns
Subtracting in columns: 352 − 128
- Ones: 2 − 8 will not work. Rename 1 ten as 10 ones. Now 12 − 8 = 4.
- Tens: 4 tens are left. 4 − 2 = 2.
- Hundreds: 3 − 1 = 2.
- 352 − 128 = 224.
Try it: subtract in columns
Word problems
- One step: the library has 645 books. 230 are out on loan. How many are on the shelves? 645 − 230 = 415.
- Two steps: Mere has 48 stickers. She gets 25 more, then gives 30 to her brother.
- 48 + 25 = 73, then 73 − 30 = 43. Mere has 43 stickers.
Common mistakes
- For 137 + 54, line up the 5 under the 1.Line up from the right: 4 under 7, and 5 under 3.
- In 352 − 128, do 8 − 2 = 6 because it is easier.Always take the bottom number from the top. Rename a ten so it is 12 − 8 = 4.
- 34 + 76 = 100.34 + 66 = 100. Count on 6 to 40, then 60 to 100.
- After renaming, leave the extra ten out.Add the renamed ten into the tens column: 3 + 5 + 1 = 9.
Your turn
1
45 + __ = 100
2
600 + 300
3
256 + 70
4
263 + 48
5
A bus has 36 people on it. 18 get off and 25 get on. How many are on the bus now?
Show answer
1. 55 2. 900 3. 326 4. 311 5. 43 (36 − 18 = 18, then 18 + 25 = 43)
What to take away
- Use small facts to solve big ones.
- Line up the places, starting from the ones.
- Ten of one place renames as one of the next.
Report a mistake
More in Number
Numbers to 1,000: Hundreds, Tens and OnesNotesCounting in Groups, Rounding and EstimatingNotesSkip Counting, Odd and Even NumbersNotesAdding and Subtracting up to 1,000NotesMultiplying and Dividing with GroupsNotesUnit Fractions: One Equal PartNotesComparing Fractions and Finding Equal OnesNotesAdding Fractions and Finding Fractions of NumbersNotesDollars, Cents and Giving ChangeNotes
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