Boats and Streams - General Questions

A man rows at 10 km/hr in still water. The river flows at 2 km/hr. How long will he take to row 6 km upstream and come back?

A man takes 3 times as long to row a distance upstream as to row the same distance downstream. If the speed of the boat in still water is 20 km/hr, what is the speed of the stream?

A man takes 5 times as long to row a distance upstream as to row the same distance downstream. If the speed of the boat in still water is 15 km/hr, what is the speed of the stream?

A man takes 7 times as long to row a distance upstream as to row the same distance downstream. If the speed of the boat in still water is 4 km/hr, what is the speed of the stream?

A boat whose speed in still water is 10 km/hr goes 28 km downstream. If the stream flows at 4 km/hr, how long does it take?