Adding and Taking Away to 100
Primary 2 · Mathematics
Adding and Taking Away to 100
Lesson 2 of 3
12 min
Mutesi sold 27 mandazi on Monday and 15 on Tuesday. She wants the total.
She starts with the ones: 7 + 5 = 12. Twelve! But only one digit fits in the ones column.
Do not worry. There is a clever swap, and today you learn it.
Think of beans. Ten loose beans can be tied together into one bundle of ten.
Tying them does not make more beans. It is the same beans, just tied up. That is all carrying is: a swap, not magic.
Step 1. Add the ones. 7 + 5 = 12 ones.
Step 2. Swap. 12 ones is the same as 1 ten and 2 ones.
Step 3. Write and carry. Write the 2 in the ones column. Send the 1 ten next door: write a small 1 above the tens column.
Step 4. Add the tens. 2 tens + 1 ten + the carried 1 ten = 4 tens.
4 tens and 2 ones is 42. So 27 + 15 = 42. Mutesi sold 42 mandazi.
| Tens | Ones | |
|---|---|---|
| carried | 1 | |
| 2 | 7 | |
| 1 | 5 | |
| 4 | 2 |
Ones: 8 + 6 = 14. Swap it: 14 is 1 ten and 4 ones. Write 4, carry 1.
Tens: 4 + 2 + 1 = 7.
Answer: 74.
The 9 is ones, so put it under the 6.
Ones: 6 + 9 = 15. Write 5, carry 1 ten.
Tens: 3 + 1 = 4.
Answer: 45.
Look at your ones answer. Is it 10 or more? Then carry. Is it 9 or less? Then there is nothing to carry.
Try 34 + 52. Ones: 4 + 2 = 6. Six is small, so no carrying. Tens: 3 + 5 = 8. Answer: 86.
Ones: 7 + 8 = 15. Write 5, carry 1.
Tens: 5 + 3 + 1 = 9.
Answer: 95. Look how close to 100 you got!
Tip
Ten ones are too many for one house. Tie them into one bundle and send the bundle next door, to the tens.
The carried 1 is small, but never forget it. Write it above the tens column before you add that column.
Do each one on paper, then tap Show answer to check.
19 + 13 = ?
Ones: 9 + 3 = 12. Write 2, carry 1. Tens: 1 + 1 + 1 = 3. Answer: 32.
45 + 28 = ?
Ones: 5 + 8 = 13. Write 3, carry 1. Tens: 4 + 2 + 1 = 7. Answer: 73.
34 + 57 = ?
Ones: 4 + 7 = 11. Write 1, carry 1. Tens: 3 + 5 + 1 = 9. Answer: 91.
66 + 7 = ?
Ones: 6 + 7 = 13. Write 3, carry 1. Tens: 6 + 1 = 7. Answer: 73.
At umuganda, Habimana fills a sack with 26 plastic bottles. Umutoni fills hers with 39. How many bottles did they collect altogether?
26 + 39. Ones: 6 + 9 = 15. Write 5, carry 1. Tens: 2 + 3 + 1 = 6. Answer: 65 bottles.
What you learned today
Quick check
Ones: 8 + 5 = 13. What do you write in the ones?
