Explanation:

A boat takes a total of 2 hours to cover 9 km downstream and return, given that the speed of the stream is 6 km/h. What is the speed of the boat in still water?

Let the speed of the boat = x km/h

Speed of current = 6 km/h

Speed upstream = (x— 6) km/h

Speed downstream = (x + 6) km/h

Time upstream + Time downstream = 2 hours 
9/(x-6)+ 9/(x+6)=2 
x = 12 
Hence, The Required speed of boat in still water is 12 Km/h.