Lesson 5.4 · Adding and Subtracting Tens
Subtracting tens
If you can find 7 − 3, you can find 70 − 30. Just think in tens, the same way you did for adding.
Take away tens
70 is 7 tens. 30 is 3 tens.
7 tens − 3 tens = 4 tens.
4 tens is 40. So 70 − 30 = 40.
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The 3 white rows are taken away. 4 green rows of ten are left.
Subtract tens from tens
Subtract the number of tens. Then say it in tens.
: 9 tens − 5 tens = 4 tens = 40.
Use adding to help
Adding and subtracting are partners. To find 80 − 20, ask: 20 + ___ = 80?
2 tens + 6 tens = 8 tens. So 80 − 20 = 60.
Worked example: 60 − 40
6 tens − 4 tens = 2 tens.
2 tens is 20. So 60 − 40 = 20.
Worked example: Count back by tens
Find 90 − 30.
Start at 90. Count back 3 tens: 80, 70, 60.
So 90 − 30 = 60.
Worked example: 50 − 50
5 tens − 5 tens = 0 tens.
So 50 − 50 = 0.
Common mistake
Say the answer in tens. 80 − 20 is 6 tens, which is 60. It is not 6!
Tip
Check with adding. If 70 − 30 = 40, then 40 + 30 should be 70. It is!
Practice
What is 50 − 20?
Type a number.
What is 40 − 30?
Type a number.
What is 90 − 20?
Type a number.
What is 80 − 60?
Type a number.
What is 70 − 40?
A box has 60 crayons. Ben takes out 10 crayons. How many crayons are left in the box?
Type a number.
Fill in the blank: 90 − ___ = 50.
Type a number.
There are 80 kids at the park. 50 kids go home. How many kids are still at the park?
Type a number.