a) 40 minutes
b) 1 hour
c) 1 hr 15 min
d) 1 hr 30 min
Answer: c
Explanation:
$$\eqalign{
& {\text{Rate}}\,{\text{downstream}} \cr
& = \left( {\frac{1}{{10}} \times 60} \right)km/hr = 6\,km/hr \cr
& {\text{Rate}}\,{\text{upstream}} = 2\,km/hr \cr
& {\text{Speed}}\,{\text{in}}\,{\text{still}}\,{\text{water}} \cr
& = \frac{1}{2}\left( {6 + 2} \right)km/hr = 4\,km/hr \cr
& {\text{Required}}\,{\text{time}} = {\frac{5}{4}} \,hrs \cr
& = 1\frac{1}{4}hrs \cr
& = 1\,hr\,15\,\min . \cr} $$
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