2022 CSAT, all questions
UPSC Prelims 2022 · CSAT
Question 29: Logical Reasoning and Mental Ability
In the series AABABCABCDABCDE.., which letter appears at the 100th place?
aG
bH
cI
dJ
AnswerUPSC’s answer is (c).
The series is built in growing blocks: A, AB, ABC, ABCD, ABCDE and so on, with block n holding n letters. After n blocks the letters used total n(n + 1)/2: 13 blocks give 91 and 14 blocks give 105. The 100th letter therefore lies in block 14 (A to N), at place 100 − 91 = 9, which is the ninth letter, I.
Why each option is right or wrong
- (a) G is the 7th letter of block 14.
- (b) H is the 8th letter of block 14.
- (c) Correct: the 9th letter is I.
- (d) J is the 10th letter of block 14.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2022, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.