a) 2 kmph
b) 2.2 kmph
c) 2.5 kmph
d) 4 kmph
Answer: a
Explanation: Let downstream speed be D and Upstream speed be U. Then
$$\eqalign{
& {\frac{{48}}{U} + \frac{{72}}{D}} = 12\,.\,.\,.\,.\,.\left( 1 \right) \cr
& {\frac{{72}}{U} + \frac{{48}}{D}} = 13\,.\,.\,.\,.\,.\left( 2 \right) \cr} $$
On solving the equation (1) and (2),
D = 12 kmph
U = 8 kmph
Thus, speed of the current
$$ = \frac{{D – U}}{2} = \frac{4}{2} = 2\,{\text{kmph}}$$
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