There are two banks of river of which distance is 21 km. A motorboat takes 270 min in going and coming back one bank to another bank. It takes 40 min to take passenger from second bank. Find speed of boat in still water if flow of river is 2.5 km/hr?

11.5 km/hr

Let speed of boat in still water is U km/hr

Speed of boat in downstream = (U + 2.5) km/hr

Speed of boat in upstream = (U - 2.5) km/hr

ATQ,

\(\frac{21}{U\;+\;2.5}\) + \(\frac{21}{U\;-\;2.5}\) = 230 min.

Note : Go with the options, check and satisfy

U = 11.5 km/hr satisfy the equation