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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