UPSC Prelims 2009 Q71: Science & Technology – Logical Reasoning & Puzzles
- (a) A and F
- (b) B and D
- (c) B and F
- (d) None of the above
Answer: (c) B and F
Explanation
A row-arrangement logic puzzle: who stands at the extremes?
Six persons (A, B, C, D, E, F) stand in a row, with specific adjacency and positional clues.
| Clue | Deduction |
|---|---|
| C and D stand close together, alongside E | C, D, E occupy three consecutive positions, with C-D adjacent and E next to one of them |
| B stands beside A only | B has exactly one neighbour, meaning B is at a row end, with A in the adjacent seat |
| A is fourth from F | The positional gap between A and F is 4 (3 people between them) |
Working out the arrangement: Placing B at one end (position 1) and A at position 2 (B's only neighbour), the gap-of-4 rule places F at position 6, the row's other end, since the positional difference between A (2) and F (6) is exactly 4. The remaining positions 3, 4, 5 then hold C, D, E as a consecutive block, satisfying the first clue.
Resulting row:
| Position | Occupant |
|---|---|
| 1 | B |
| 2 | A |
| 3, 4, 5 | C, D, E (in some order) |
| 6 | F |
Answer: (c) B and F.
Why each option is right or wrong
- (a) Incorrect. A sits next to B, not at a row end, so 'A and F' is wrong.
- (b) Incorrect. B and D are not both at the row's extremes in this arrangement.
- (c) CORRECT. Working through all the clues places B and F at the two ends of the row.
- (d) Incorrect. The puzzle is solvable, with B and F uniquely determined as the extremes.
Key point
Placing B at one end (with A as its only neighbour) and using the 'A is fourth from F' gap-of-4 rule places F at the other end — making B and F the two people standing at the extremes.
Remember
Row puzzle: B and F stand at the two extremes, with A beside B and C-D-E filling the middle three seats.
Common mistakes
- Misreading 'fourth from F' as a smaller or larger positional gap than 4.
- Not recognising that B's 'beside A only' clue forces B to a row end (since only end positions have exactly one neighbour).
- Failing to check that the remaining positions for C, D, E form a valid consecutive block.
Exam tip
'X stands beside Y only' in a row puzzle = X is at an END position, with Y as its single neighbour. Use this to anchor the rest of the arrangement.
Sources
- R. S. Aggarwal — A Modern Approach to Verbal and Non-Verbal Reasoning (Puzzle Test)
- Arun Sharma — How to Prepare for Logical Reasoning for the CAT (arrangement and attribute-matching puzzles)