a) 1 hour 24 minutes
b) 2 hour 21 minutes
c) 2 hour 25 minutes
d) 3 hour
Answer: a
Explanation:
$$\eqalign{
& {\text{Speed upstream}} \cr
& {\text{ = }}\left( {\frac{{56}}{{1\frac{3}{4}}}} \right)km/hr \cr
& = \left( {56 \times \frac{4}{7}} \right)km/hr \cr
& = 32km/hr \cr
& {\text{let speed downstream be }}x{\text{ km/hr}}{\text{.}} \cr
& {\text{Then speed of boat in still water }} \cr
& {\text{ = }}\frac{1}{2}\left( {x + 32} \right)km/hr \cr
& {\text{ }}\frac{1}{2}\left( {x + 32} \right) = 36\,\,\, \Rightarrow x = 40 \cr
& {\text{Hence , required time}} \cr
& {\text{ = }}\left( {\frac{{50}}{{40}}} \right)hrs \cr
& = 1\frac{2}{5}hrs \cr
& = 1\,{\text{hour}}\,24\operatorname{minutes} \cr} $$
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