CA Foundation · Quantitative Aptitude · Number Series, Coding-Decoding and Odd Man Out
In the series 4, 9, 19, 39, 79, 159, one term is wrong, but the rest follow a single rule. Which term is wrong?
No term is wrong. Each term equals twice the previous term plus 1: 4 to 9, 9 to 19, 19 to 39, 39 to 79 and 79 to 159. Since every step obeys this rule, the series is fully consistent.
- A19
- B39
- C79
- DNone; the series is entirely consistentCorrect
Explanation
Each term is double the previous term plus 1 only if we check: 4×2+1=9, 9×2+1=19, 19×2+1=39, 39×2+1=79, 79×2+1=159. Every step fits, so no term is wrong. Options naming 19, 39 or 79 are wrong because each of these terms follows the rule.
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