Primary 2 Addition and Subtraction
Use place value to add and subtract within 1000. Follow the columns from ones to thousands and see when a ten or a hundred must be regrouped.
Add within 1000
Join the ones first, then tens, then hundreds. When a column makes 10 or more, write its ones digit and carry 1 into the next place. A total of 1000 puts 1 in the Thousands column.
234 + 152 = ?
| Thousands | Hundreds | Tens | Ones | |
|---|---|---|---|---|
| Regroup | ||||
| 2 | 3 | 4 | ||
| + | 1 | 5 | 2 | |
| Answer |
Look at the columns. What would you calculate first?
Common mistake: In 346 + 238, writing 14 in the Ones column. Write 4 ones and regroup the other 10 ones as 1 ten.
Ask: “What do the ones make? Where should the extra ten go? Does the answer need a thousands digit?”
Subtract within 1000
Start at the ones. If the top digit is too small, regroup one unit from the place to its left. A zero may mean you must go farther left first, then work back toward the ones. Compare 1000 minus a one-digit, two-digit or three-digit number to see which places need regrouping.
586 − 243 = ?
| Thousands | Hundreds | Tens | Ones | |
|---|---|---|---|---|
| Regroup 2 | ||||
| Regroup 1 | ||||
| Regroup 3 | ||||
| 5 | 8 | 6 | ||
| − | 2 | 4 | 3 | |
| Answer |
Look at the columns. What would you calculate first?
Common mistake: In 704 − 268, the 0 tens cannot give a ten straight to the 4 ones. Regroup 1 hundred as 10 tens first, then regroup 1 ten as 10 ones.
Ask: “Which place can give us a unit to regroup? What is the top number worth after regrouping?”
Check the answer
Addition and subtraction can check each other. After you find a total, subtract one part from it. You should get the other part. The total stays the same even when its places are regrouped.
Ask: “If this is the whole, which part could we subtract to check the answer?”