a) 1 km/hr
b) 1.5 km/hr
c) 2 km/hr
d) 2.5 km/hr
Answer: a
Explanation: Suppose he move 4 km downstream in x hours
Speed downstream = $$\frac{4}{x}$$ km/hr
Speed upstream = $$\frac{3}{x}$$ km/hr
$$\eqalign{
& \therefore \frac{{48}}{{\left( {4/x} \right)}} + \frac{{48}}{{\left( {3/x} \right)}} = 14\,or\,x = \frac{1}{2} \cr} $$
So, Speed downstream = 8 km/hr
Speed upstream = 6 km/hr
Rate of the stream = $$\frac{1}{2}$$(8 – 6) km/hr
= 1 km/hr
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