A boat goes 9 km for an hour in still water. It takes twice as much time for the boat to go upstream as it takes while going downstream. Find the speed of the current?

Let boat speed in still water = b = 9 km/hr, current speed = c. Downstream speed = b+c, upstream speed = b-c. Time upstream = 2 × time downstream. Distance upstream = distance downstream (let it be d). So d/(b-c) = 2d/(b+c). Cross-multiplying: d(b+c) = 2d(b-c). b+c = 2b-2c. 3c = b. Therefore current speed c = 9/3 = 3 km/hr. This matches option C.