Ajay can row his boat in still water at a speed of 11 km/hr. If the speed of the stream is 7 km/hr, how long will it take to cover a distance of 54 km downstream?

3 hours 

Speed of boat in downstream = Speed of boat in still water + Speed of stream

                             = 11 + 7 = 18 km/hr

Time taken to cover 54 km downstream = $\frac{distance}{Speed\;downstream}$

                            = $\frac{54}{18}$ = 3 hours