UPSC Prelims 1997 · General Studies
Question 147: Basic Numeracy
A boat which has a speed of 5 km/hr in still water crosses a river of width 1 km along the shortest possible path in 15 minutes. The velocity of the river water in km/hr is
AnswerThe answer is (b).
To cross by the shortest path the boat must head upstream at an angle so that the river's push is cancelled and it moves straight across. The crossing speed is 1 km in 15 minutes, that is 4 km/h. The boat's 5 km/h is the hypotenuse, so (river speed)² = 5² - 4² = 9, and the river's speed is 3 km/h.
Why each option is right or wrong
- (a) 1 km/h would give a crossing speed of about 4.9 km/h, not 4.
- (b) Right: 3 km/h.
- (c) 4 km/h is the crossing speed, not the river's.
- (d) Read as the square root of 41, it comes from adding 25 and 16 instead of subtracting.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 1997, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.