What if the ones add up to 10 or more? Then you make a ten, just like with facts, and add tens and ones as before.
Key idea
If the ones make 10 or more, trade 10 ones for 1 new ten. $27 + 6$: $7 + 6 = 13$ ones, which is 1 ten and 3 ones. So the answer is 33.
Why it works
10 ones and 1 ten are the same amount. Trading keeps the total the same, but lets us write it with one digit in each place.
Worked example: A new ten
What is $27 + 6$?
- Ones: $7 + 6 = 13$.
- 13 ones is 1 ten and 3 ones.
- Tens: $2 + 1 = 3$ tens. The answer is 33.
Answer: 33
Worked example: Exactly ten ones
What is $58 + 2$?
- Ones: $8 + 2 = 10$. That is 1 ten and 0 ones.
- Tens: $5 + 1 = 6$ tens.
- $58 + 2 = 60$
Answer: 60
Common mistake
Writing 213 for $27 + 6$, putting the 13 ones next to the 2 tens. Each place can hold only one digit. And 213 is far too big: $27 + 6$ must be in the 30s.
For grown-ups: Play a trading game with pennies and dimes: whenever you have 10 pennies, trade them for a dime.