Finding a missing value from the mean

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1
The mean of 5 numbers is 5. 4 of them are 10, 3, 3, 3. What is the last one?
2
The mean of 5 numbers is 6. 4 of them are 7, 9, 1, 1. What is the last one?
3
The mean of 5 numbers is 20. 4 of them are 6, 21, 21, 23. What is the last one?
4
The mean of 5 numbers is 12. 4 of them are 15, 17, 8, 6. What is the last one?
5
The mean of 5 numbers is 10. 4 of them are 19, 3, 11, 5. What is the last one?
6
The mean of 5 numbers is 10. 4 of them are 19, 9, 9, 3. What is the last one?
7
The mean of 5 numbers is 17. 4 of them are 7, 20, 13, 21. What is the last one?
8
The mean of 5 numbers is 15. 4 of them are 7, 14, 8, 14. What is the last one?
9
The mean of 5 numbers is 11. 4 of them are 11, 14, 7, 9. What is the last one?
10
The mean of 5 numbers is 10. 4 of them are 13, 13, 4, 7. What is the last one?
11
The mean of 5 numbers is 10. 4 of them are 8, 5, 17, 1. What is the last one?
12
The mean of 5 numbers is 12. 4 of them are 8, 13, 12, 8. What is the last one?
13
The mean of 5 numbers is 16. 4 of them are 11, 7, 25, 12. What is the last one?
14
The mean of 5 numbers is 18. 4 of them are 31, 17, 15, 1. What is the last one?
15
The mean of 5 numbers is 6. 4 of them are 6, 7, 3, 1. What is the last one?
16
The mean of 5 numbers is 7. 4 of them are 1, 9, 10, 5. What is the last one?
17
The mean of 5 numbers is 8. 4 of them are 11, 4, 11, 2. What is the last one?
18
The mean of 5 numbers is 11. 4 of them are 14, 9, 3, 13. What is the last one?
19
The mean of 5 numbers is 10. 4 of them are 3, 13, 8, 12. What is the last one?
20
The mean of 5 numbers is 7. 4 of them are 11, 1, 3, 5. What is the last one?

What is in this set

The numbers in this set run from 1 to 38.
A careful solution takes 3 written steps on average.
The mistake this set invites most often, in 98% of the problems: the mean was written down again; the missing number is the total the mean implies, less what is already there.
Answers are a whole number.

Drawing 240 problems from this page produced 239 different ones, 100% — so a sheet of 20 is a new sheet almost every time.

The same skill, harder or easier

We would put this in front of a grade 7 class. That is our judgement, not a standard, and it is printed here so you can disagree with it. Everything else we put in that year.