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UPSC Prelims 2025 CSAT, question 30

Topic: Series and patterns

Q30

Consider the sequence

AB_CC_A_BCCC_BBC_C that follows a certain pattern. Which one of the following completes the sequence ?

  1. (a) B, C, B, C, A
  2. (b) A, C, B, C, A
  3. (c) B, C, B, A, CCorrect answer
  4. (d) C, B, B, A, C

Solution.

Answer: (c) B, C, B, A, C

  1. The 18-letter string divides into three identical 6-letter blocks ABBCCC (ABBCCC + ABBCCC + ABBCCC).
  2. Matching AB_CC_A_BCCC_BBC_C against ABBCCCABBCCCABBCCC fills the five blanks at positions 3, 6, 8, 13, and 17 with B, C, B, A, C.