9514 1404 393
Answer:
10, 12, 14 and maybe {8, 10, 12} and {12, 14, 16}, depending on the meaning of the question
Step-by-step explanation:
If x is the middle one, then their sum is 3x. You want ...
15 < 3x/2 < 21 . . . . assuming "between 15 and 21" does not include 15 or 21
30 < 3x < 42
10 < x < 14
The only even integer in that range is 12, so the three integers are ...
10, 12, 14 . . . . . half their sum is 18
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We have used <, rather than ≤ in the above solution. This assumes that "between" does not include the values at the end of the range. If those values are included, then two additional solutions are possible: {8, 10, 12} and {12, 14, 16}. For the first set, the half-sum is 15; for the second set, the half-sum is 21.
