Q. A man rows a boat 18 kilometers in 4 hours downstream and returns upstream in 12 hours. The speed of the (in km/hr) stream is :
Answer: 1.5 km/hr
Notes: Rate downstream = $latex \frac{18}{4}=\frac{9}{2} km/hr&s=1$ Rate upstream = $latex \frac{18}{12}=\frac{3}{2} km/hr&s=1$ Now, speed of the stream = $latex \frac{Rate Downstream - Rate Upstream }{2}&s=1$ $latex =>\frac{\frac{9}{2}-\frac{3}{2}}{2}=\frac{6}{4}=\frac{3}{2}&s=2$ $latex =>1.5 km/hr$ Hence option [B] is the right answer.

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