The speed of a boat downstream is 15 km/hr and the speed of current is 3 km/hr. Find the total time taken by the boat to cover 15 km upstream and 15 km downstream.

Explanation:-

$$\eqalign{ & {\text{Given}} \cr & {\text{Speed of boat in downstream}} \cr & {\text{ = 15 km/h}} \cr & {\text{Speed of current = 3 km/h}} \cr & {\text{Speed of boat in still water}} \cr & {\text{ = 12 km/h}} \cr & {\text{Time taken at upstream }} \cr & {\text{ = }}\frac{{15}}{{12 - 3}} = \frac{{15}}{9}h = 1{\text{h }}\,40\min \cr & {\text{Time taken at downstream}} \cr & {\text{ = }}\frac{{15}}{{12 + 3}} = 1{\text{h}}\, \cr & {\text{total time }}\,{\text{ = 2 h 40 min }} \cr}$$