Addition and Subtraction to 1000
Primary 3 · Mathematics
Addition and Subtraction to 1000
Lesson 1 of 3
13 min
On umuganda day, Mugisha's village made bricks. In the morning they made 247 bricks. In the afternoon they made 128 more.
The village leader asks, "How many bricks do we have now?" Mugisha's fingers are not enough for this one!
Today you will learn the neat, safe way to add big numbers: in columns, one place at a time.
Every 3-digit number lives in three places: hundreds, tens and ones.
In 247 there are 2 hundreds, 4 tens and 7 ones.
When you add in columns, ones must sit under ones, tens under tens, and hundreds under hundreds. If the digits are not lined up straight, the answer is wrong before you even begin.
| H | T | O |
|---|---|---|
| 2 | 3 | 4 |
| + 1 | 5 | 2 |
| 3 | 8 | 6 |
Ones: 4 + 2 = 6. Write 6.
Tens: 3 + 5 = 8. Write 8.
Hundreds: 2 + 1 = 3. Write 3.
So 234 + 152 = 386.
Notice where you started: on the right, in the ones column. Always. Never start with the hundreds.
Ones: 7 + 8 = 15. Now there is a problem. Only one digit is allowed to live in the ones column, and 15 needs two!
Think about it this way: 15 ones is the same as 1 ten and 5 ones. The 5 ones stay at home in the ones column. The 1 ten does not belong there, so it walks next door to the tens column.
Moving that ten next door is called carrying. Write a small 1 above the tens column so you do not forget it.
| H | T | O | |
|---|---|---|---|
| carry | 1 | ||
| 2 | 4 | 7 | |
| + | 1 | 2 | 8 |
| 3 | 7 | 5 |
Tens: 4 + 2 + 1 (the carried ten) = 7. Write 7.
Hundreds: 2 + 1 = 3. Write 3.
So 247 + 128 = 375. The village made 375 bricks.
Ones: 5 + 2 = 7. Nothing to carry. Write 7.
Tens: 6 + 7 = 13. But 13 tens is the same as 1 hundred and 3 tens. The 3 tens stay. The 1 hundred walks next door into the hundreds column.
Write 3 in the tens, and carry 1 to the hundreds.
Hundreds: 1 + 1 + 1 (the carried hundred) = 3. Write 3.
So 165 + 172 = 337.
Ones: 6 + 5 = 11. Write 1, carry 1.
Tens: 8 + 7 + 1 = 16. Write 6, carry 1.
Hundreds: 3 + 2 + 1 = 6. Write 6.
So 386 + 275 = 661.
Two carries, and nothing went wrong! You never added big scary numbers. You only ever added one small column at a time.
Tip
Ten is too big to stay in one house. Each column can only hold a digit from 0 to 9. As soon as a column makes 10 or more, the ten packs its bag and moves to the column on its left.
Write the carried 1 above the next column straight away, before you forget it. Forgetting the carry is the number one mistake in P3!
Copy each one into columns in your book. Start with the ones. Then tap Show answer to check.
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