Students practice finding the missing number in vertical addition problems involving 2-digit and 1-digit numbers.

I'll solve each problem step by step, finding the missing addend in each addition equation.

Let me work through all 20 problems:

Problem 1: 54 + ___ = 54
- To find the missing number: 54 - 54 = 0
- Answer: 0

Problem 2: 66 + ___ = 1
- Wait, this doesn't make sense. Let me re-read... it should be 66 + ___ = something that ends in 1
- Looking at the format, it's likely: 66 + ___ = (a two-digit number ending in 1)
- Actually, I need to see what the sum should be. Let me reconsider the layout.
- The answer line shows "1" which means the ones digit is 1
- So: 66 + ___ = _1
- 6 + ? = 1 (in ones place), so we need 6 + 5 = 11, carry the 1
- 6 + 1 (carried) = 7, so the tens digit would be 7
- Therefore: 66 + 5 = 71
- Answer: 5

Problem 3: 35 + ___ = 2
- Following the same pattern: 35 + ___ = _2
- 5 + ? = 2 (in ones place), so 5 + 7 = 12, carry the 1
- 3 + 1 (carried) = 4
- Therefore: 35 + 7 = 42
- Answer: 7

Problem 4: 17 + ___ = 18
- 17 + ? = 18
- 18 - 17 = 1
- Answer: 1

Problem 5: 80 + 8 = ___
- 80 + 8 = 88
- Answer: 88

Problem 6: 4 + 1 = ___
- 4 + 1 = 5
- Answer: 5

Problem 7: ___ + 1 = 72
- ? + 1 = 72
- 72 - 1 = 71
- Answer: 71

Problem 8: ___ + 5 = 7
- ? + 5 = 7
- 7 - 5 = 2
- Answer: 2

Problem 9: ___ + 1 = 31
- ? + 1 = 31
- 31 - 1 = 30
- Answer: 30

Problem 10: ___ + 7 = 28
- ? + 7 = 28
- 28 - 7 = 21
- Answer: 21

Problem 11: 75 + ___ = 78
- 75 + ? = 78
- 78 - 75 = 3
- Answer: 3

Problem 12: 34 + ___ = 37
- 34 + ? = 37
- 37 - 34 = 3
- Answer: 3

Problem 13: 72 + ___ = 74
- 72 + ? = 74
- 74 - 72 = 2
- Answer: 2

Problem 14: 42 + 3 = ___
- 42 + 3 = 45
- Answer: 45

Problem 15: 19 + 0 = ___
- 19 + 0 = 19
- Answer: 19

Problem 16: ___ + 1 = 46
- ? + 1 = 46
- 46 - 1 = 45
- Answer: 45

Problem 17: ___ + 0 = 17
- ? + 0 = 17
- 17 - 0 = 17
- Answer: 17

Problem 18: 12 + ___ = 17
- 12 + ? = 17
- 17 - 12 = 5
- Answer: 5

Problem 19: 20 + ___ = 28
- 20 + ? = 28
- 28 - 20 = 8
- Answer: 8

Problem 20: 64 + 2 = ___
- 64 + 2 = 66
- Answer: 66

Let me verify my calculations one more time:

1. 54 + 0 = 54 ✓
2. 66 + 5 = 71 ✓
3. 35 + 7 = 42 ✓
4. 17 + 1 = 18 ✓
5. 80 + 8 = 88 ✓
6. 4 + 1 = 5 ✓
7. 71 + 1 = 72 ✓
8. 2 + 5 = 7 ✓
9. 30 + 1 = 31 ✓
10. 21 + 7 = 28 ✓
11. 75 + 3 = 78 ✓
12. 34 + 3 = 37 ✓
13. 72 + 2 = 74 ✓
14. 42 + 3 = 45 ✓
15. 19 + 0 = 19 ✓
16. 45 + 1 = 46 ✓
17. 17 + 0 = 17 ✓
18. 12 + 5 = 17 ✓
19. 20 + 8 = 28 ✓
20. 64 + 2 = 66 ✓

All calculations check out!

Final Answer:
1) 0
2) 5
3) 7
4) 1
5) 88
6) 5
7) 71
8) 2
9) 30
10) 21
11) 3
12) 3
13) 2
14) 45
15) 19
16) 45
17) 17
18) 5
19) 8
20) 66