A man rows a boat downstream in 3 hour & returns back the same distance upstream in 6 hours. If the speed of stream is 2 km/hr, then the speed of the boat in still water is:

6 km/hr

Let the speed of the boat be x km/h.

& speed of the stream = 2 km/h

Down stream speed = x + 2 km/h

Up stream speed = x – 2 km/h

Down stream time = 3 hours

Up stream time = 6 hours

Distance for both is same.

$∵ \frac{D}{x+2}=3$ ⇒ D = 3x + 6

& $\frac{D}{x-2}=6$ ⇒ D = 6x - 12

∴ 6x - 12 = 3x + 6  ⇒ 3x = 18

x = 6 km/h