Explanation
Speed of boat in upstream = 15\ km/hr
Distance = 1.5\ km
Time = t_1=\dfrac {1.5}{15} = 6\ min
Distance move by float in this time
5\times \dfrac {1}{10}=\dfrac {1}{2}\ km
Speed of boat w.r.t. float,
20 km/hr , t_2= \dfrac {2\ km}{20} = 6\ min.
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