UPSC Prelims 2016 · CSAT
Question 9: Logical Reasoning and Mental Ability
A person X was driving in a place where all roads ran either north-south or east-west, forming a grid. Roads are at a distance of 1 km from each other in a parallel. He started at the intersection of two roads, drove 3 km north, 3 km west and 4 km south. Which further route could bring him back to his starting point, if the same route is not repeated ?
AnswerUPSC’s answer is (b).
Take the start as (0, 0), east as x and north as y. 3 km north gives (0, 3); 3 km west gives (-3, 3); 4 km south gives (-3, -1). To return to (0, 0) he needs net 3 km east and 1 km north, which is option (b). Check the others: (a) ends at (0, -3); (c) ends at (-5, 0); (d) ends at (-3, -3). Remember: track coordinates, not directions.
Why each option is right or wrong
- (a) Ends at (0, -3), not the start.
- (b) Right: 3 km east then 1 km north reaches (0, 0) without retracing.
- (c) Ends at (-5, 0).
- (d) Ends at (-3, -3).
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2016, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.