Question : To go a distance of 144 km upstream, a rower takes 12 hours while it takes her only 9 hours to row the same distance downstream. What is the speed of the stream?
Option 1: 3 km/hr
Option 2: 1 km/hr
Option 3: 1.5 km/hr
Option 4: 2 km/hr
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Correct Answer: 2 km/hr
Solution :
Given: Distance = 144 km
And the time taken to row upstream = 12 hours
And the time taken to row downstream = 9 hours
So, speed of boat in upstream = $\frac{144}{12}$ = 12 km/hr
And speed of boat in downstream = $\frac{144}{9}$ = 16 km/hr
Let x be the speed of the boat in still water and y be the speed of the stream.
So, the speed of the boat in upstream = x – y
⇒ x – y = 12-----------(i)
And the speed of the boat in downstream = x + y
⇒ x + y = 16------------(ii)
Subtracting (i) from (ii), we get:
2y = 4
$\therefore$ y = 2 km/hr
Hence, the correct answer is 2 km/hr.
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