Q. A boat goes 20 km downstream in one hour and the same distance upstream in two hours. The speed of the boat in still water is:
Answer: 15 km/hr
Notes: Let the speed of boat in still water be x km/hr and the rate of stream be y km/hr. ∴ Downstream rate = (x + y) km/hr and upstream rate = (x – y) km/hr $ Now, \frac{20}{x + y} = 1$ $ => x + y = 20 …….\left ( 1 \right )$ $ and \frac{20}{x-y} = 2$ $ => x – y = 10 …….\left ( 2 \right )$ From (1) and (2) we have x = 15 km/hr. Hence option [D] is correct answer.