In still water, the speed of a motorboat is 15 km/h. The boat travels 30 km downstream and returns in 4 hours and 30 minutes. The stream's speed (in km/h) is:
Answer & explanation
Correct answer: option 3
We know that,
Downstream = Speed of boat in still water + Speed of stream/current
Upstream = Speed of boat in still water - Speed of stream/current
Given,
The speed of motorboat = 15 km/hr
The total time to go 30 km downstream and comes back is 4 hours 30 minutes.
Let the speed of stream be a.
The total time is 4 + \(\frac{30}{60}\) = \(\frac{9}{2}\)
= \(\frac{30}{(15 - a) }\) = \(\frac{9}{2}\)
= \(\frac{30}{(15^2 – a^2) }\) = \(\frac{3}{20}\)
= 225 - a2 = 200
= a = \(\sqrt {25}\) = 5 km/hr