UPSC Prelims 2026 · CSAT
Question 18: Logical Reasoning and Mental Ability
A pattern formed by two characters a and b is repeated more than once in the following string :
| ×b×a×a××a×a×bab |
What is ×× in the 7th and 8th positions from the left in the above string?
AnswerUPSC’s answer is (d).
The string has 15 characters, so try a block of 5 repeated three times (position n matches n + 5 and n + 10). Positions 6 and 11 are a, so position 1 is a. Position 2 is b, so 7 and 12 are b. Position 13 is b, so 3 and 8 are b. Positions 4, 9 and 14 are a, and position 15 is b, so 5 and 10 are b. The block is abbab, so positions 7 and 8 are b and b.
Why each option is right or wrong
- (a) Wrong: position 7 is b, not a.
- (b) Wrong: position 8 is b, not a.
- (c) Wrong: position 7 is b and position 8 is also b.
- (d) Right: the block abbab puts b at positions 7 and 8.
Only the provisional key (27 May 2026) was found; the final key replaces it when found.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2026, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.