Number series – find the wrong term (identify the outlier) Series: 125, 126, 124, 127, 123, 129
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A126
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B124
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C123
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D129
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E125
Answer
Correct Answer: 129
Explanation
Introduction / Context:This is a classic alternating-increment/decrement pattern. The rule adds and subtracts consecutive integers in turn; one number breaks the rhythm.
Given Data / Assumptions:
- Series: 125, 126, 124, 127, 123, 129.
- Exactly one wrong term.
Concept / Approach:Check if the series alternates between “+k” and “−(k+1)” for k = 1, 2, 3, … A standard form is: +1, −2, +3, −4, +5, −6, …
Step-by-Step Solution:
125 → 126: +1 ✔126 → 124: −2 ✔124 → 127: +3 ✔127 → 123: −4 ✔Next should be +5: 123 + 5 = 128, but the list has 129 ✖Verification / Alternative check:Continuing the pattern after the correction would require the next step to be −6. Since only one term must be wrong, 129 is the outlier (should have been 128).
Why Other Options Are Wrong:
- 126, 124, 123 all fit the alternating +1, −2, +3, −4 pattern exactly; removing any of them ruins the clean alternation.
Common Pitfalls:Trying a fixed-difference rule; ignoring that alternating increase/decrease with step sizes 1, 2, 3, 4, … is a well-used pattern in reasoning tests.
Final Answer:129