Length of river is 40 km. Speed of stream is 3 km/hr. Boat takes 6 hours more to return than to go in river. Find the speed of boat in still water.

7 km/hr

Let speed of boat in still water = u km/hr

Speed of boat in downstream = (u + 3) km/hr

Speed of boat in upstream = (u - 3) km/hr

ATQ,

\(\frac{40}{u\;-\;3}\) - \(\frac{40}{u\;+\;3}\) = 6

Now, check from option, nearest value which can divide 40 easily when adding and subtracting with 3.

Hence, 7 is the option which satisfy the equation.