Add a Two-Digit Number and a One-Digit Number | 1st Grade Math

Add a Two-Digit Number and a One-Digit Number - Grade 1 Math
Grade 1 Math  ›  Operations with Place Value  ›  Lesson 19
Grade 1 Math - Operations with Place Value

Add a Two-Digit Number and a One-Digit Number

Learn how to add a two-digit number and a one-digit number within 100 using tens and ones, base-ten thinking, counting on, and make-the-next-ten strategies.

When the ones stay below 10, the tens remain unchanged. When the ones make 10 or more, compose 10 ones into a new ten and add that ten to the tens place.

2-digit + 1-digit addition Addition within 100 Tens and ones Compose a ten 1st Grade Math

๐ŸŽฏ What You Will Learn

Add a one-digit number to the ones
Keep track of the tens while combining the ones.
Recognize when no new ten is needed
If the ones total less than 10, the tens digit stays the same.
Compose a new ten when needed
If the ones total 10 or more, trade 10 ones for 1 new ten.
Use more than one strategy
Solve with base-ten blocks, counting on, place value, or making the next ten.
For parents & teachers — standards alignment: This lesson directly addresses CCSS 1.NBT.C.4, which explicitly includes adding a two-digit number and a one-digit number within 100. Students use concrete models, drawings, and place-value strategies, relate their reasoning to written equations, and compose a new ten when the ones total 10 or more. The lesson also builds on CCSS 1.NBT.B.2 place-value understanding. Texas Grade 1 TEKS 1.2(B) and 1.2(C) support the place-value models and representations used here. TEKS 1.3(A) specifically targets adding a multiple of 10 and a one-digit number, so the broader two-digit + one-digit cases in this lesson are additional Common Core-aligned practice for Texas students rather than a direct Grade 1 TEKS expectation.

๐Ÿง  Key Vocabulary

two-digit number — a number from 10 through 99 made from tens and ones.
one-digit number — a number from 0 through 9.
ones — single units in the ones place.
tens — groups of 10 ones.
compose a ten — make 1 new ten from 10 ones.
sum — the result of addition.

๐Ÿ”ข First Look at the Ones

When you add a one-digit number to a two-digit number, the new amount begins in the ones place. For example, in 24 + 5, the 2 tens stay in place while the 4 ones and 5 ones are combined.

24 + 5
2 tens stay 2 tens
4 + 5 = 9 ones
Result
2 tens
9 ones

24 + 5 = 29

No new ten: if the ones add to 9 or less, simply keep the tens and write the new number of ones.

๐Ÿ”Ÿ When the Ones Make a New Ten

In a problem such as 36 + 7, the ones make 6 + 7 = 13. Ten of those ones can be composed into 1 new ten.

36 + 4 = 40
→
40 + 3 = 43

Another way to see it is 13 ones = 1 ten + 3 ones. The 3 tens in 36 become 4 tens, and 3 ones remain. So 36 + 7 = 43.

๐Ÿ› ️ Four Useful Strategies

Base-ten blocks Build the two-digit number, add ones, and trade 10 ones for a ten if needed.
Count on Start at the two-digit number and count on the one-digit addend.
Make the next ten Break apart the one-digit addend so you can reach the next multiple of 10 first.
Use place value Combine the ones and then decide whether the tens stay the same or increase by 1.
Common mistake: do not add the one-digit number to the tens digit. In 36 + 7, the 7 represents ones, so it combines with the 6 ones first.

๐ŸŒŸ 25 Worked Examples

Try each problem first. Then open “Show solution” to compare your strategy.
1 Add ones without making a new ten
Find the sum: 21 + 3 = □.
Show solution Hide solution

21 has 2 tens and 1 one. Add 3 more ones: 1 + 3 = 4. The 2 tens stay the same, so 21 + 3 = 24.

2 Keep the tens, add the ones
Find the sum: 34 + 2 = □.
Show solution Hide solution

34 has 3 tens and 4 ones. Add 2 to the ones: 4 + 2 = 6. Keep the 3 tens. The sum is 36.

3 Use place value
Find the sum: 52 + 4 = □.
Show solution Hide solution

52 is 5 tens and 2 ones. Add 4 ones to 2 ones: 2 + 4 = 6. The tens do not change, so the sum is 56.

4 Count on from the two-digit number
Find the sum: 63 + 5 = □.
Show solution Hide solution

Start at 63 and count on 5: 64, 65, 66, 67, 68. So 63 + 5 = 68.

5 Add to a multiple of ten
Find the sum: 70 + 6 = □.
Show solution Hide solution

70 is 7 tens and 0 ones. Add 6 ones. The result is 7 tens and 6 ones, or 76.

6 Add only in the ones place
Find the sum: 45 + 3 = □.
Show solution Hide solution

45 has 4 tens and 5 ones. Add 3 ones: 5 + 3 = 8. Keep the 4 tens. The sum is 48.

7 Build the new ones digit
Find the sum: 81 + 7 = □.
Show solution Hide solution

81 has 8 tens and 1 one. Add 7 ones to make 8 ones. The sum is 88.

8 Use a quick known fact
Find the sum: 26 + 2 = □.
Show solution Hide solution

Use the ones fact 6 + 2 = 8. Keep the 2 tens. Therefore 26 + 2 = 28.

9 Add ones to a number ending in zero
Find the sum: 40 + 8 = □.
Show solution Hide solution

40 has 4 tens and no ones. Add 8 ones to get 48.

10 Keep the tens unchanged
Find the sum: 57 + 2 = □.
Show solution Hide solution

The ones are 7 + 2 = 9. Since 9 is less than 10, no new ten is made. Keep the 5 tens. The sum is 59.

11 Use expanded thinking
Find the sum: 32 + 5 = □.
Show solution Hide solution

Think 32 = 30 + 2. Then 2 + 5 = 7. Combine 30 + 7 to get 37.

12 Add without composing a ten
Find the sum: 74 + 4 = □.
Show solution Hide solution

The ones are 4 + 4 = 8. Keep the 7 tens. So 74 + 4 = 78.

13 Compose a new ten
Find the sum: 28 + 5 = □.
Show solution Hide solution

The ones are 8 + 5 = 13. Thirteen ones is 1 ten and 3 ones. Add the new ten to the 2 tens: 3 tens and 3 ones = 33.

14 Make the next ten
Find the sum: 36 + 7 = □.
Show solution Hide solution

36 needs 4 to reach 40. Break 7 into 4 + 3. First 36 + 4 = 40, then 40 + 3 = 43.

15 Compose a ten with base-ten thinking
Find the sum: 47 + 6 = □.
Show solution Hide solution

7 ones + 6 ones = 13 ones. Trade 10 ones for 1 ten, leaving 3 ones. Now there are 5 tens and 3 ones: 53.

16 Cross into the next ten
Find the sum: 58 + 4 = □.
Show solution Hide solution

58 needs 2 to make 60. Break 4 into 2 + 2. Then 58 + 2 = 60 and 60 + 2 = 62.

17 Use the next-ten strategy
Find the sum: 69 + 3 = □.
Show solution Hide solution

69 needs 1 to reach 70. Break 3 into 1 + 2. Then 69 + 1 = 70 and 70 + 2 = 72.

18 Add 8 by making a new ten
Find the sum: 75 + 8 = □.
Show solution Hide solution

75 needs 5 to reach 80. Break 8 into 5 + 3. Then 75 + 5 = 80 and 80 + 3 = 83.

19 Compose a ten in the 80s
Find the sum: 84 + 7 = □.
Show solution Hide solution

4 ones + 7 ones = 11 ones. Compose 10 ones into 1 new ten. Then 8 tens become 9 tens, with 1 one left: 91.

20 Add 9 by making the next ten
Find the sum: 26 + 9 = □.
Show solution Hide solution

26 needs 4 to reach 30. Break 9 into 4 + 5. Then 26 + 4 = 30 and 30 + 5 = 35.

21 Cross a ten from 39
Find the sum: 39 + 5 = □.
Show solution Hide solution

39 needs 1 to reach 40. Break 5 into 1 + 4. Then 39 + 1 = 40 and 40 + 4 = 44.

22 Use 10 as a bridge
Find the sum: 56 + 8 = □.
Show solution Hide solution

56 needs 4 to reach 60. Break 8 into 4 + 4. Then 56 + 4 = 60 and 60 + 4 = 64.

23 Compose from 7 + 7
Find the sum: 67 + 7 = □.
Show solution Hide solution

7 + 7 = 14 ones. Compose 10 ones into 1 new ten and keep 4 ones. Six tens become 7 tens, so the sum is 74.

24 Add 9 efficiently
Find the sum: 48 + 9 = □.
Show solution Hide solution

48 needs 2 to reach 50. Break 9 into 2 + 7. Then 48 + 2 = 50 and 50 + 7 = 57.

25 Show two strategies
Find the sum: 57 + 8 = □. Show two ways to think.
Show solution Hide solution

Place-value way: 7 + 8 = 15 ones, so compose a ten and get 6 tens 5 ones = 65. Next-ten way: 57 + 3 = 60, then add the remaining 5 to get 65.

๐ŸŽฎ Independent Practice — 30 Problems

  1. Find the sum: 22 + 4 = □.
  2. Find the sum: 31 + 6 = □.
  3. Find the sum: 43 + 5 = □.
  4. Find the sum: 54 + 4 = □.
  5. Find the sum: 62 + 7 = □.
  6. Find the sum: 71 + 3 = □.
  7. Find the sum: 80 + 9 = □.
  8. Find the sum: 25 + 4 = □.
  9. Find the sum: 46 + 3 = □.
  10. Find the sum: 53 + 6 = □.
  11. Find the sum: 64 + 2 = □.
  12. Find the sum: 72 + 5 = □.
  13. Find the sum: 83 + 4 = □.
  14. Find the sum: 90 + 7 = □.
  15. Find the sum: 27 + 5 = □.
  16. Find the sum: 38 + 4 = □.
  17. Find the sum: 49 + 3 = □.
  18. Find the sum: 56 + 7 = □.
  19. Find the sum: 68 + 5 = □.
  20. Find the sum: 74 + 9 = □.
  21. Find the sum: 85 + 8 = □.
  22. Find the sum: 29 + 6 = □.
  23. Find the sum: 37 + 8 = □.
  24. Find the sum: 48 + 7 = □.
  25. Find the sum: 59 + 9 = □.
  26. Find the sum: 66 + 8 = □.
  27. Find the sum: 78 + 6 = □.
  28. Find the sum: 94 + 6 = □.
  29. Solve 46 + 7 by making the next ten. Show how you break apart 7.
  30. Solve 52 + 9 using a base-ten or place-value explanation.

๐Ÿ† Lesson Test — 15 Questions

  1. Find the sum: 23 + 5 = □.
  2. Find the sum: 42 + 6 = □.
  3. Find the sum: 55 + 3 = □.
  4. Find the sum: 61 + 7 = □.
  5. Find the sum: 73 + 5 = □.
  6. Find the sum: 82 + 6 = □.
  7. Find the sum: 91 + 4 = □.
  8. Find the sum: 24 + 8 = □.
  9. Find the sum: 35 + 9 = □.
  10. Find the sum: 46 + 8 = □.
  11. Find the sum: 57 + 5 = □.
  12. Find the sum: 68 + 7 = □.
  13. Find the sum: 79 + 4 = □.
  14. Solve 58 + 7. Explain how making the next ten helps.
  15. Solve 86 + 9. Explain what happens to the ones.

๐Ÿ” Complete Answer Key

Show practice answer key Hide practice answer key

Practice Answers

1. 26
2. 37
3. 48
4. 58
5. 69
6. 74
7. 89
8. 29
9. 49
10. 59
11. 66
12. 77
13. 87
14. 97
15. 32
16. 42
17. 52
18. 63
19. 73
20. 83
21. 93
22. 35
23. 45
24. 55
25. 68
26. 74
27. 84
28. 100
29. 53. Break 7 into 4 + 3: 46 + 4 = 50, then 50 + 3 = 53.
30. 61. 52 has 5 tens and 2 ones. Add 9 ones: 2 + 9 = 11 ones = 1 ten and 1 one, so the result is 6 tens and 1 one.
Show test answer key Hide test answer key

Test Answers

1. 28
2. 48
3. 58
4. 68
5. 78
6. 88
7. 95
8. 32
9. 44
10. 54
11. 62
12. 75
13. 83
14. 65. Add 2 to reach 60, then add the remaining 5.
15. 95. The ones make 6 + 9 = 15, so 10 ones become 1 new ten and 5 ones remain.

๐Ÿ’ก Common Mistakes to Watch For

Adding to the tens instead of the ones
A one-digit addend represents ones, so begin in the ones place.
Missing the new ten
If the ones make 10 or more, compose 10 ones into 1 additional ten.
Counting from 1
Start from the two-digit number and count on only the amount being added.
Using a shortcut without understanding
Be able to explain the sum with tens and ones, not only write the answer.
What comes next: Lesson 20 focuses on adding and subtracting multiples of 10 using place-value reasoning.

๐Ÿ”— Related Lessons

Lesson 18 — Add Within 100 Using Tens and Ones Review the general place-value ideas behind addition within 100. Lesson 2 — Tens and Ones: Understanding Place Value Review how two-digit numbers are built from tens and ones.

๐Ÿ–จ️ Printable Companion

A future Two-Digit Plus One-Digit Addition Practice Pack can include base-ten models, no-new-ten practice, compose-a-ten problems, next-ten strategy activities, mixed review, and a complete answer key.

← Lesson 18: Add Within 100 Using Tens and Ones Grade 1 Math Hub Next: Add & Subtract Multiples of 10 →

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