UPSC Prelims 2010 Q78: Science & Technology – Logical Reasoning & Puzzles
In a group of five persons A, B, C, D and E, there is a professor, a doctor and a lawyer. A and D
are unmarried ladies, and do not work. Of the married couple in the group, E is the husband. B is
the brother of A and is neither a doctor nor a lawyer. Who is the professor?
- (a) B
- (b) C
- (c) A
- (d) Cannot be determined with the available data
Answer: (a) B
Explanation
A family logic puzzle: who is the professor?
Five people — A, B, C, D, E — include a professor, a doctor, and a lawyer among them.
| Clue | Deduction |
|---|---|
| A and D are unmarried ladies who do not work | So A and D hold none of the three professions; the professor, doctor, and lawyer must be among B, C, E |
| E is the husband of the married couple | Confirms E is a working, married man |
| B is the brother of A, and is neither a doctor nor a lawyer | Since the three professions are split among B, C, and E, and B is excluded from doctor and lawyer, B must be the remaining profession: professor |
Working through the logic:
| Person | Status |
|---|---|
| A, D | Unmarried, don't work — excluded from all three professions |
| B, C, E | The working three, holding professor/doctor/lawyer among them |
| B | Neither doctor nor lawyer → must be professor |
Answer: (a) B.
Why each option is right or wrong
- (a) CORRECT. Since the three professions belong only to B, C, and E, and B is explicitly not a doctor or lawyer, B must be the professor.
- (b) Incorrect. C could be the doctor or lawyer, but the clue specifically identifies B, by elimination, as the professor.
- (c) Incorrect. A is explicitly stated to be unmarried and not working, so A cannot hold any of the three professions.
- (d) Incorrect. The puzzle can be determined: working through the given clues uniquely identifies B as the professor.
Key point
Since A and D are unmarried and don't work, the three professions (professor, doctor, lawyer) belong only to B, C, and E — and since B is explicitly neither doctor nor lawyer, B must be the PROFESSOR by elimination.
Remember
Family puzzle: A and D don't work, so professions belong to B, C, E; B is neither doctor nor lawyer, so B is the professor.
Common mistakes
- Forgetting that A and D are excluded from holding any profession, narrowing the pool to just B, C, and E.
- Not applying elimination logic correctly: B being 'neither doctor nor lawyer' directly implies B is the professor.
- Assuming the puzzle is underdetermined when it is in fact fully solvable from the given clues.
Exam tip
First identify who CAN'T hold any profession (the non-working, unmarried ladies A and D), then apply elimination among the remaining three (B, C, E).
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)