Difficulty: Medium
Correct Answer: 39
Explanation:
Pattern insight
The sequence alternates between “add an increasing odd number” and “multiply by an increasing integer”.
Detail
Start at 2: +1 → 3; ×1 → 3; +2 → 5; ×2 → 10; +3 → 13; ×3 → 39; +4 → 43; ×4 → 172; +5 → 177.
Result
The missing term after 13 is 13 × 3 = 39.
Common pitfall
Assuming a single arithmetic or geometric pattern; it is a mixed-operation pattern.
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