UPSC Prelims 2017 · CSAT
Question 76: Logical Reasoning and Mental Ability
Eight railway stations A, B, C, D, E, F, G and H are connected either by two-way passages or one-way passages. One-way passages are from C to A, E to G, B to F, D to H, G to C, E to C and H to G. Two-way passages are between A and E, G and B, F and D, and E and D. While travelling from C to H, which one of the following stations must be passed through?
AnswerUPSC’s answer is (b).
Start at C. C's only road out leads to A, and A links onward only to E, so every route from C passes through E. The routes are C-A-E-D-H, or the longer C-A-E-G-B-F-D-H. G and F are skipped on the short route, so they are not compulsory. E is the station the key picks. Always find the forced first steps from the start before testing options.
Why each option is right or wrong
- (a) G can be skipped by going E to D to H.
- (b) Correct: C's only exit leads to A, whose only onward link is E.
- (c) D is also on every route, being the only road into H, but the key goes with E.
- (d) F can be skipped on the short route.
Worth knowing: Station D is also on every route from C to H, since the D to H road is the only way into H, so (c) is equally defensible; the key gives E.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.