UPSC Prelims 2010 · General Studies
Question 27: Basic Numeracy
Two trains leave New Delhi at the same time. One travels north at 60 kmph and the other travels south at 40 kmph. After how many hours will the trains be 150 km apart ?
AnswerThe answer is (a).
The trains leave the same point in opposite directions, so the distance between them grows at the sum of their speeds: 60 + 40 = 100 km per hour. Time to be 150 km apart = 150 / 100 = 3/2 hours, that is 1.5 hours. Remember: for opposite directions add the speeds; for the same direction subtract them.
How you could have got this
No shortcut here: in a “how many” question, ruling out options doesn’t help. Every statement has to be judged.
This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Right: 150 / 100 = 3/2 hours.
- (b) 4/3 hour would give only about 133 km at 100 km/h.
- (c) 3/4 hour would give only 75 km.
- (d) 15/2 hours would be right only for a closing speed of 20 km/h.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2010, General Studies, Series A. The paper on upsc.gov.in.