Lesson 5.2 · Adding and Subtracting Tens
Adding tens
What is 30 + 40? It looks big, but it is as easy as 3 + 4. The trick is to think in tens.
Tens are like ones
30 is 3 tens. 40 is 4 tens.
3 tens + 4 tens = 7 tens.
7 tens is 70. So 30 + 40 = 70.
It works just like 3 + 4 = 7. You are adding tens instead of ones.
Add tens to tens
To add tens, add the number of tens. Then say it in tens.
: 5 tens + 2 tens = 7 tens = 70.
Adding tens to any number
You can add tens to a number like 23, too. Only the tens change.
23 is 2 tens and 3 ones. Add 30 (3 tens).
2 tens + 3 tens = 5 tens. The 3 ones stay the same.
So 23 + 30 = 53.
Worked example: 20 + 60
2 tens + 6 tens = 8 tens.
8 tens is 80. So 20 + 60 = 80.
Worked example: 41 + 20
41 has 4 tens and 1 one. 20 is 2 tens.
4 tens + 2 tens = 6 tens. The 1 one stays.
So 41 + 20 = 61.
Worked example: Count on by tens
Find 36 + 40.
Start at 36. Count on 4 tens: 46, 56, 66, 76.
So 36 + 40 = 76.
Common mistake
Add tens to tens. Don't add the tens to the ones! In 23 + 30, the 3 tens join the 2 tens. The answer is 53, not 26.
Practice
What is 20 + 30?
Type a number.
What is 40 + 50?
Type a number.
What is 70 + 30?
Type a number.
What is 25 + 20?
Type a number.
What is 18 + 60?
Type a number.
Which one equals 60?
Sam has 34 blocks. He gets 3 bags with 10 blocks in each bag. How many blocks does he have now?
Type a number.
Fill in the blank: 52 + ___ = 92.
Type a number.