A boat while dowenstream in a reiver converd a distance of 50 miles at an average speed of 60 miles per hour. While returning , because of the water resistance , it took 1 hour 15 minutes to cover the same distance What was the average speed during the whole journey?

$$\eqalign{ & {\text{Time taken to cover 50 miles downstream}} \cr & {\text{ = }}\left( {\frac{{50}}{{60}}} \right)hr{\text{ = }}\frac{5}{6}hr.{\text{ }} \cr & {\text{Time taken to cover 50 miles upstream}} \cr & {\text{ = 1hr 15m = 1}}\frac{1}{4}hrs = \frac{5}{4}hrs \cr & {\text{Total time taken to cover 100 miles}} \cr & {\text{ = }}\left( {\frac{5}{6} + \frac{5}{4}} \right)hrs = \frac{{25}}{{12}}hrs \cr & \therefore {\text{Average speed }} \cr & {\text{ = }}\frac{{100}}{{\left( {\frac{{25}}{{12}}} \right)}}mph \cr & = \left( {\frac{{100 \times 12}}{{25}}} \right)mph \cr & = 48mph. \cr} $$