a) 40 mph
b) 48 mph
c) 50 mph
d) 55 mph
Answer: b
Explanation:
$$\eqalign{
& {\text{Time taken to cover 50 miles downstream}} \cr
& {\text{ = }}\left( {\frac{{50}}{{60}}} \right)hr{\text{ = }}\frac{5}{6}hr.{\text{ }} \cr
& {\text{Time taken to cover 50 miles upstream}} \cr
& {\text{ = 1hr 15m = 1}}\frac{1}{4}hrs = \frac{5}{4}hrs \cr
& {\text{Total time taken to cover 100 miles}} \cr
& {\text{ = }}\left( {\frac{5}{6} + \frac{5}{4}} \right)hrs = \frac{{25}}{{12}}hrs \cr
& {\text{Average speed }} \cr
& {\text{ = }}\frac{{100}}{{\left( {\frac{{25}}{{12}}} \right)}}mph \cr
& = \left( {\frac{{100 \times 12}}{{25}}} \right)mph \cr
& = 48mph. \cr} $$
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