Lesson 3.1 · Adding and Subtracting Within 100
Adding two-digit numbers
You already know how to add tens. Now you can add any two numbers like 23 and 45. Just add the tens, then add the ones.
Break each number apart
23 is 2 tens and 3 ones. 45 is 4 tens and 5 ones.
| Tens | Ones | |
|---|---|---|
| 23 | 2 | 3 |
| 45 | 4 | 5 |
| Total | 6 | 8 |
Tens: 2 + 4 = 6 tens. That is 60.
Ones: 3 + 5 = 8 ones. That is 8.
Put them together: 60 + 8 = 68. So 23 + 45 = 68.
Tens with tens, ones with ones
To add two-digit numbers:
- Add the ones.
- Add the tens.
- Put the tens and ones together.
Stack them up
You can also write one number on top of the other. Line up the ones under the ones. Line up the tens under the tens.
Add the ones column first: 3 + 5 = 8. Then the tens column: 2 + 4 = 6.
Worked example: 52 + 36
Ones: 2 + 6 = 8.
Tens: 5 + 3 = 8 tens, which is 80.
80 + 8 = 88. So 52 + 36 = 88.
Worked example: Hop on a number line
Find 41 + 27.
27 is 2 tens and 7 ones. Start at 41. Hop 20 to get to 61. Then hop 7 to get to 68.
So 41 + 27 = 68.
Worked example: Adding a number with 0 ones
Find 30 + 64.
Ones: 0 + 4 = 4.
Tens: 3 + 6 = 9 tens, which is 90.
90 + 4 = 94.
Common mistake
Line up the numbers by place. In 52 + 6, the 6 is ones. It goes under the 2, not under the 5. The answer is 58, not 112.
Tip
Check your answer by adding in the other order. 45 + 23 is also 68.
Practice
What is 32 + 14?
Type a number.
What is 26 + 43?
Type a number.
What is 50 + 37?
Type a number.
What is 61 + 25?
Type a number.
What is 44 + 55?
Type a number.
Which one equals 75?
A jar has 35 red beads and 42 blue beads. How many beads are in the jar?
Type a number.
Fill in the blank: 23 + ___ = 58.
Type a number.