A boat can go 60 km downstream and 40 km upstream in 12 hours 30 minutes. It can go 84 km downstream and 63 km upstream in 18 hours 54 minutes. What is the speed (in km/h, to the nearest integer) of the boat in still water?

The speed of a motorboat in still water is 15 km/h. It travels 30 km downstream and returns to the starting point in a total time of 4 hours 30 minutes. What is the speed of the stream (in km/h)?

The speed of a motorboat in still water is 15 km/h. It travels 30 km downstream and returns to the starting point in a total time of 4 hours and 30 minutes. What is the speed of the stream (in km/h)?