What you need to know
When we add two-digit numbers, we add the ones first and then the tens. Sometimes the ones add up to 10 or more. When that happens we regroup (your teacher might say trade or carry): 10 ones become 1 ten, and we move that ten into the tens column.
Regrouping only makes sense if you remember what each digit is worth. In 47, the 4 means 4 tens (40) and the 7 means 7 ones.
| Number | Tens | Ones |
|---|
| 47 | 4 | 7 |
| 25 | 2 | 5 |
| 60 | 6 | 0 |
| 18 | 1 | 8 |
47 + 25 in columns
- Write 47 on top and 25 underneath, with ones under ones and tens under tens.
- Add the ones: 7 + 5 = 12. That is 1 ten and 2 ones.
- Write the 2 in the ones column and carry the 1 ten to the top of the tens column.
- Add the tens: 4 + 2 + the carried 1 = 7 tens.
- Answer: 47 + 25 = 72. Check: 47 + 20 = 67, then 67 + 5 = 72.
38 + 6 (adding a one-digit number)
- Add the ones: 8 + 6 = 14. That is 1 ten and 4 ones.
- Write 4 in the ones column and carry the 1 ten.
- Add the tens: 3 + 1 = 4 tens.
- Answer: 38 + 6 = 44. Check: 38 + 2 = 40, then 40 + 4 = 44.
Another way: the jump strategy. You can also add in your head by jumping along a number line. Add the tens first, then the ones. This is a great way to check a column answer.
56 + 27 using the jump strategy
- Start at 56.
- Jump 20 (the tens in 27): 56 + 20 = 76.
- Jump 4 to reach the next ten: 76 + 4 = 80.
- Jump the 3 that is left (because 4 + 3 = 7): 80 + 3 = 83.
- Answer: 56 + 27 = 83.
Common mistake: writing the whole 12 in the ones column, so 47 + 25 becomes 612. Only one digit can live in each column. The 1 ten has to move across to the tens column.
Common mistake: forgetting to add the carried ten. In 47 + 25, leaving out the carried 1 gives 62 instead of 72. Write the little 1 above the tens column as soon as you regroup so you cannot lose it.
Check yourself
- 29 + 14 = ?
- 36 + 48 = ?
- 57 + 8 = ?
- 65 + 27 = ?
- 18 + 18 = ?
- Does 43 + 25 need regrouping? Answer yes or no, then find the total.
Answers: 1. 43 2. 84 3. 65 4. 92 5. 36 6. No (3 + 5 = 8, which is less than 10); the total is 68.