A pattern formed by two characters a and b is repeated more than once in the following string : x b x a x a x x a x a x bab What is x x in the 7th and 8th positions from the left in the above string?
- (a) aa
- (b) ab
- (c) ba
- (d) bb ✓ UPSC's answer
Why the answer is (d)
• The string has 15 characters: x b x a x a x x a x a x b a b (x = unknown).
• A pattern repeated a whole number of times must have length 3 or 5. Length 3 fails: positions 4 and 13 would have to match, but they are a and b.
• Length 5 works. Reading positions 1, 6, 11 gives a; 2, 7, 12 gives b; 3, 8, 13 gives b; 4, 9, 14 gives a; 5, 10, 15 gives b. The pattern is "abbab", repeated three times: abbab abbab abbab.
• Positions 7 and 8 are b and b: (d).
Why the other options are wrong
- (a) aa
- Position 7 must equal position 2, which is b.
- (b) ab
- Position 7 is b, not a.
- (c) ba
- Position 8 must equal position 13, which is b.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2026, held on 24 May 2026. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.