Practicing Success
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: |
4 7 5 3 |
5 |
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 |