UPSC Prelims 2010 Q6: Science & Technology – Logical Reasoning & Puzzles
- (a) Red
- (b) Black
- (c) White
- (d) Blue
Answer: (c) White
Explanation
A cuboid colour-logic puzzle
Six sides, six colours, with given adjacency/opposite relationships; we must deduce what's opposite brown.
| Clue | Deduction |
|---|---|
| Red opposite Black | Red-Black form one opposite pair |
| Blue adjacent to White | Blue and White are not opposite each other |
| Brown adjacent to Blue | Brown and Blue are not opposite each other |
| Red face down | Red/Black pair is the vertical pair, leaving Blue, White, Brown, and the sixth colour as the four side faces |
Since Blue is adjacent to both White and Brown, and only one remaining colour can be truly opposite Brown among the side faces, elimination leaves White as the side not adjacent to Brown, making it the one opposite Brown.
Cuboid logic — opposite faces:
| Pair | Relationship |
|---|---|
| Red - Black | Opposite (given) |
| Blue - White | Adjacent (not opposite) |
| Brown - Blue | Adjacent (not opposite) |
| Brown - White | By elimination, opposite |
Answer: (c) White.
Why each option is right or wrong
- (a) Incorrect. Red is already paired opposite Black, so it cannot also be opposite Brown.
- (b) Incorrect. Black is paired opposite Red, not Brown.
- (c) CORRECT. By elimination (Red-Black fixed as opposite; Blue adjacent to both White and Brown), White must be opposite Brown.
- (d) Incorrect. Blue is explicitly stated to be adjacent to Brown, so it cannot be opposite it.
Key point
With Red-Black as a fixed opposite pair, and Blue adjacent to both White and Brown, elimination leaves WHITE as the face opposite Brown.
Remember
Cuboid puzzle: Red-Black fixed opposite; Blue adjacent to White and Brown; by elimination, White is opposite Brown.
Common mistakes
- Forgetting that adjacent faces can never be the same as opposite faces in a cuboid puzzle.
- Not using the given opposite pair (Red-Black) as a fixed anchor before deducing the rest.
- Rushing to a plausible-sounding answer without systematically eliminating adjacency clues.
Exam tip
In cuboid puzzles, fix the given opposite pair first, then use adjacency clues to eliminate options for the unknown pair by process of elimination.
Sources
- R. S. Aggarwal — A Modern Approach to Verbal and Non-Verbal Reasoning (Cubes and Dice)
- Arun Sharma — How to Prepare for Logical Reasoning for the CAT (cubes and dice puzzles)