Q. A man takes half time in rowing a certain distance downstream than upstream. What is the ratio or the speed in still water to the speed of current?
(a) 1:2
(b) 2:1
(c) 1:3
(d) 3:1
Correct Answer: (d) 3:1
Question from UPSC Prelims 2020 CSAT Paper
Explanation :
PROBLEM: Find ratio of man’s speed in still water to current speed
FORMULAS:
Time = Distance/Speed
– Downstream time = Distance/(v + c)
– Upstream time = Distance/(v – c)
SOLUTION:
1. Speeds:
– Downstream = v + c
– Upstream = v – c
2. Since downstream time is half:
v + c = 2(v – c)
v + c = 2v – 2c
-v + 3c = 0
v = 3c
3. Therefore:
Man’s speed : Current speed
= v : c
= 3 : 1
ANSWER: The ratio of man’s speed in still water to current speed is 3:1
