If a boat goes upstream at a speed of 24 km/hr and comes back the same distance at 40 km/hr. What is the average speed (in km/hr) for the total journey?

Speed of a boat in still water is 12 kmph and that of the current is 3 kmph. A man row a boat upstream up to 135 km and return downstream to the starting point. Find the total time taken for the entire journey in hours.

A motorboat whose speed is 20 km/h in still water takes 30 minutes more to go 24 km upstream than to cover the same distance downstream. If the speed of the boat in still water is increased by 2 km/h, then bow much time will it take to go 39 km downstream and 30 km upstream?