a) 4 km/hr
b) 5 km/hr
c) 6 km/hr
d) 10 km/hr
Answer: b
Explanation: Let the speed of the stream be x km/hr
Speed downstream = (15 + x) km/hr
Speed upstream = (15 – x) km/hr
$$\eqalign{
& \frac{{30}}{{ {15 + x} }} + \frac{{30}}{{ {15 – x} }} = 4\frac{1}{2} \cr
& \frac{{900}}{{225 – {x^2}}} = \frac{9}{2} \cr
& 9{x^2} = 225 \cr
& {x^2} = 25 \cr
& x = 5\,km/hr \cr} $$
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