Consider the sequence
A _ BCD _ BBCDABC _ DABC _ D
that follows a certain pattern. Which one of the following completes the sequence?
Answer & explanation
Answer: (c) A, A, C, D
Split the sequence into blocks of five letters. Each block is ABCD with one letter doubled, and the doubled letter moves along: AABCD, ABBCD, ABCCD, ABCDD. The blanks are A, A, C, D.
- Write it in blocks of five: A_BCD | _BBCD | ABC_D | ABC_D.
- The second block has B doubled in ABCD, so each block is ABCD with one letter doubled.
- The doubled letter moves A → B → C → D: AABCD, ABBCD, ABCCD, ABCDD.
- So the blanks are A (block 1), A (block 2), C (block 3) and D (block 4).
- Check: the completed series is AABCD ABBCD ABCCD ABCDD, a clean pattern.
Remember · In letter series with blanks, split the string into equal blocks (try the length of the repeating unit plus one).
Question and answer: UPSC's official GS Paper II (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·