The speed of a boat is thrice that of the speed of the current. The total time taken by a boatman to reach its starting point from a point 72 km farther the departure point is 10.8 hours. What is the speed of the boat in downstream?

Let current speed = c km/hr. Then boat speed = 3c km/hr. Downstream speed = 3c + c = 4c, upstream speed = 3c - c = 2c. Total time = 72/(4c) + 72/(2c) = 18/c + 36/c = 54/c = 10.8 hours. Solving: c = 5 km/hr. Downstream speed = 4c = 20 km/hr. Option A (15) would mean current = 3.75, giving time 72/18.75 + 72/7.5 = 10.56 hours, incorrect.