Four children are sitting in a row. A is occupying the seat next to B but not next to C. If C is not sitting next to D, who is/are occupying seat/seats adjacent to D ?
- (a) B
- (b) A ✓ UPSC's answer
- (c) B and A
- (d) Impossible to tell
Why the answer is (b)
• Number the four seats 1, 2, 3, 4 from left to right.
• Since A is next to B, A and B must occupy two adjacent seats; since A is not next to C, C cannot sit in a seat adjacent to A.
• Since C is not next to D, C and D must be separated by at least one seat.
• The only arrangements satisfying all conditions are D-A-B-C and C-B-A-D.
• In both arrangements, D is at an end and its only adjacent seat is occupied by A, so option (b) follows.
Why the other options are wrong
- (a) B
- B is not adjacent to D in the only valid arrangements D-A-B-C and C-B-A-D.
- (c) B and A
- D is adjacent only to A, not to B and A together, because D is at an end in both valid arrangements.
- (d) Impossible to tell
- It is possible to tell because the conditions force D to be adjacent only to A in the two valid arrangements.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2014, held on 24 August 2014. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.