- 13
- 18
- 15
- 16
- 12
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Answer: C official key
Approach: Overlap in averages: the 5th number is counted in both groups.
Solution:
Sum of 9 numbers = 9 × 15 = 135.
First five sum = 5 × 13 = 65; last five sum = 5 × 17 = 85.
65 + 85 = 150 counts the 5th number twice.
5th number = 150 − 135 = 15.
Cetking shortcut: Deviation method: first five −2 each (−10), last five +2 each (+10) → 5th = 15 + 0 = 15.
Answer: C






