Number series (find the wrong term): 1, 5, 5, 9, 7, 11, 11, 15, 12, 17

Introduction / Context:Interleaved (alternating) sequences are common in reasoning tests. Here, the even-positioned terms follow a simple repeating increment pattern, and the odd-positioned terms should mirror it. One odd-position term breaks the expected rule.

Given Data / Assumptions:

• Series: 1, 5, 5, 9, 7, 11, 11, 15, 12, 17.
• Positions: odd indices = 1, 3, 5, 7, 9; even indices = 2, 4, 6, 8, 10.
• Exactly one term is incorrect.

Concept / Approach:Split into two subsequences: Odd positions and even positions. Check if each is an arithmetic progression with a small repeating step pattern (for example, +4 then +2). Identify the first violation.

Step-by-Step Solution:Even positions: 5, 9, 11, 15, 17 → increments +4, +2, +4, +2 (a neat repeat).Odd positions: 1, 5, 7, 11, 12 → intended increments should match +4, +2, +4, +2 to stay parallel: 1→5 (+4), 5→7 (+2), 7→11 (+4), 11→13 (+2 expected). The series shows 12 instead of 13.Thus, 12 is the only term that breaks the clean alternating +4, +2 motif.

Verification / Alternative check:Correcting 12 to 13 yields two perfectly matched alternating-progressions across odd and even positions, confirming a single localized error.

Why Other Options Are Wrong:

• 9, 11, 15, 17 all fit the alternating +4, +2 increments in their respective subsequences; removing any would spoil a consistent pattern.

Common Pitfalls:

• Treating the entire list as one progression rather than two interwoven ones.

Final Answer:12