5lb 21.45 - Solution assumes ordered set, but this isn't specified

GRE Quant Problem Solving
1

Problem statement
Set N consists of the first 9 positive multiples of 3
Quantity A - The average of the first and last terms in the set
Quantity B - The average of the third and seventh terms in the set

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Solution from 5 lb:
C

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Question:
For the solution to be C, one must assume that the set is ordered ascending or descending. But this isn’t explicitly specified. So the answer is either D or I am not interpting the question correctly. I find it ambiguous. Why must one assume this is an ordered set?

2

When they say first 9 positive multiple of 3 —> they are specifying the order.

3

If they wanted to reverse the set order for the first 9, this type of grammar wouldn’t hold good. You don’t think there is any ambiguity at all?

4

No sir, its crystal clear to me (First 9 +ve multiples means → 3,6,9…,27 )