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UPSC Prelims 2026 CSAT, question 18

Topic: Series and patterns

Q18

A pattern formed by two characters aa and bb is repeated more than once in the following string :

×b×a×a××a×a×bab\times b \times a \times a \times \times a \times a \times bab

What is ××\times \times in the 7th and 8th positions from the left in the above string?

  1. (a) aa
  2. (b) ab
  3. (c) ba
  4. (d) bbCorrect answer

Solution.

Answer: (d) bb

  1. Matching the 15-character string against a repeating block shows that the 5-letter block abbab repeated three times (abbab + abbab + abbab) matches every given position: 2nd (b), 4th (a), 6th (a), 9th (a), 11th (a), and 13th to 15th (bab).
  2. In abbababbababbab, the 7th character is the 2nd letter of the second block (b) and the 8th character is the 3rd letter of the second block (b), giving bb.