A boat can tarvel 36 km upstream in 5 hours. If the speed of the stream is 2.4 kmph, how much time will the boat take to cover a distance of 78 km downstream?(in hours)

Explanation:-

$$\eqalign{ & {\text{Distance covered by a boat in 5 hours}} \cr & {\text{ = }}36{\text{ }}km \cr & {\text{Rate upstream of boat}} \cr & {\text{ = }}\,\frac{{36}}{5} = 7.2\,kmph \cr & {\text{Speed of the stream}}\, = 2.4\,kmph \cr & \therefore {\text{speed of the boat in stil water}} \cr & {\text{ = }}\left( {7.2 + 2.4} \right)kmph \cr & = 9.6\,\,kmph{\text{ }} \cr & \therefore {\text{Rate downstream of the boat}} \cr & {\text{ = }}\left( {9.6 + 2.4} \right)kmph \cr & = 12\,kmph \cr & \therefore {\text{Time taken in covering 78 }}km{\text{ distance}} \cr & {\text{ = }}\frac{{78}}{{12}} \cr & = 6.5\,hours. \cr}$$