UPSC Prelims 2024 · CSAT

Question 24: Logical Reasoning and Mental Ability

Consider the sequence A _ BCD _ BBCDABC _ DABC _ D that follows a certain pattern. Which one of the following completes the sequence?

aB, A, D, C

bB, A, C, D

cA, A, C, D

dA, A, D, C

AnswerUPSC’s answer is (c).

Split the 20 letters into four groups of five. The groups are AABCD, ABBCD, ABCCD and ABCDD: each group is ABCD plus one extra letter that repeats the letter at that group's position (A, then B, then C, then D). The blanks are therefore A (second letter), A (sixth), C (fourteenth) and D (nineteenth): A, A, C, D.

Why each option is right or wrong

(a) Wrong. B in the first blank gives ABBCD as group one, and the doubled-letter pattern breaks.
(b) Wrong. B, A, C, D puts B in the first blank, which breaks the doubling of A, B, C, D in turn.
(c) Right. It gives AABCD, ABBCD, ABCCD, ABCDD.
(d) Wrong. A, A, D, C gives group three as ABCDD and group four as ABCCD, reversing the pattern.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2024, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.