A boat man rows to a place 45 km distant and back in 20 hours. He finds that he can row 12 km with the stream in the same time as 4 km against the stream. Find the speed of the stream.

Let speed of man in still water = $$x$$ km/hr and speed of stream = $$y$$ km/hr

According to ques, time taken in 12 km downstream = Time taken 4 km upstream

=> $$\frac{12}{x+y}=\frac{4}{x-y}$$

=> $$3x-3y=x+y$$

=> $$2x=4y$$

=> $$x=2y$$ --------------(i)

Also, $$\frac{45}{x+y}+\frac{45}{x-y}=20$$

=> $$\frac{15}{y}+\frac{45}{y}=20$$

=> $$20y=60$$

=> $$y=\frac{60}{20}=3$$

$$\therefore$$ Speed of stream = 3 km/hr

=> Ans - (A)