Lesson 5.3 · Adding and Subtracting Tens
Adding a two-digit and a one-digit number
Now let's add a small number to a bigger one, like 42 + 5. The secret is simple: add ones to ones. Sometimes the ones make a new ten!
Add ones to ones
42 has 4 tens and 2 ones. Add 5 ones.
2 ones + 5 ones = 7 ones. The 4 tens stay the same.
So 42 + 5 = 47.
Ones go with ones
Add the ones digit to the ones digit. The tens stay the same, unless the ones make a new ten.
When the ones make a ten
Try 37 + 6. The ones are 7 + 6 = 13. That is too many for the ones place!
13 ones is 1 ten and 3 ones. Trade 10 ones for a new ten.
🟩🟩🟩🟩🟩🟩🟩 + 🟩🟩🟩🟩🟩🟩 → 🟩🟩🟩🟩🟩🟩🟩🟩🟩🟩 and 🟩🟩🟩
Now there are 3 tens + 1 new ten = 4 tens, and 3 ones. So 37 + 6 = 43.
Make a ten
Here is another way. Break the small number apart to get to the next ten.
37 + 6: 37 needs 3 more to make 40. Break 6 into 3 and 3.
37 + 3 = 40. Then 40 + 3 = 43.
Worked example: 63 + 4
3 ones + 4 ones = 7 ones. The 6 tens stay.
63 + 4 = 67.
Worked example: 28 + 5
28 needs 2 more to make 30. Break 5 into 2 and 3.
28 + 2 = 30. Then 30 + 3 = 33.
28 + 5 = 33.
Worked example: 56 + 9
56 needs 4 more to make 60. Break 9 into 4 and 5.
56 + 4 = 60. Then 60 + 5 = 65.
56 + 9 = 65.
Common mistake
Don't add the small number to the tens! 42 + 5 is 47, not 92. The 5 is ones, so it goes with the ones.
Practice
What is 32 + 3?
Type a number.
What is 71 + 7?
Type a number.
What is 46 + 4?
Type a number.
What is 17 + 7?
Type a number.
What is 85 + 7?
Type a number.
What is 54 + 3?
Lily read 23 pages. Then she read 8 more pages. How many pages did she read in all?
Type a number.
What is 9 + 52?
Type a number.