UPSC Prelims 2021 · CSAT
Question 65: Logical Reasoning and Mental Ability
In the series _b_a_ba_b_abab_aab; fill in the six blanks ( _ ) using one of the following given four choices such that the series follows a specific order.
AnswerUPSC’s answer is (d).
The fixed letters suggest a block of six repeating. Taking the block abbaab, the filled series is abbaab abbaab abbaab, and every fixed letter in the printed series matches it. Reading off the blanks in order, at positions 1, 3, 5, 8, 10 and 15, gives a, b, a, b, a, b, which is ababab. Remember: for repeating-series puzzles, test a block length that fits the given letters, then read off the blanks.
Why each option is right or wrong
- (a) bababa gives bbaabbaabb then abab, so the order changes midway.
- (b) baabba gives bbaaab at the start, which does not repeat.
- (c) bbaabb gives bbbaab first and aabbab second, so the blocks differ.
- (d) Right: ababab completes the repeating block abbaab three times.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.