Lesson 3.3 · Adding and Subtracting Within 100
Subtracting two-digit numbers
You have 68 stickers and give away 25. How many are left? You can subtract two-digit numbers the same way you add them. Take away the ones, then take away the tens.
Break each number apart
68 is 6 tens and 8 ones. 25 is 2 tens and 5 ones.
| Tens | Ones | |
|---|---|---|
| 68 | 6 | 8 |
| take away 25 | 2 | 5 |
| Left | 4 | 3 |
Ones: 8 − 5 = 3.
Tens: 6 − 2 = 4 tens. That is 40.
40 + 3 = 43. So 68 − 25 = 43.
Ones from ones, tens from tens
To subtract two-digit numbers:
- Subtract the ones.
- Subtract the tens.
- Put the tens and ones together.
Always put the bigger number first.
Stack them up
Put the bigger number on top. Line up the places.
Worked example: 86 − 42
Ones: 6 − 2 = 4.
Tens: 8 − 4 = 4 tens, which is 40.
40 + 4 = 44. So 86 − 42 = 44.
Worked example: Count back on a number line
Find 75 − 30.
30 is 3 tens. Start at 75. Hop back 10 three times: 65, 55, 45.
So 75 − 30 = 45. Only the tens changed.
Worked example: Check with adding
Find 59 − 34, then check.
Ones: 9 − 4 = 5. Tens: 5 − 3 = 2. So 59 − 34 = 25.
Check: 25 + 34 = 59. It matches!
Common mistake
Subtract the ones from the ones. In 47 − 3, the 3 comes off the 7 ones. The answer is 44, not 17.
Tip
To check a subtraction, add your answer to the number you took away. You should get the number you started with.
Practice
What is 58 − 23?
Type a number.
What is 77 − 40?
Type a number.
What is 66 − 15?
Type a number.
What is 94 − 61?
Type a number.
What is 89 − 47?
Type a number.
Which one equals 21?
There are 48 birds in a tree. 26 birds fly away. How many birds are still in the tree?
Type a number.
Fill in the blank: 75 − ___ = 32.
Type a number.