a) 5 hours
b) 6.5 hours
c) 5.5 hours
d) 8 hours
Answer: b
Explanation: Distance covered by a boat in 5 hours = 36 km
Rate upstream of boat = $$\frac{{36}}{5}$$ = 7.2 kmph
Speed of the stream = 2.4 kmph
Speed of the boat in still water
= (7.2 + 2.4) kmph
= 9.6 kmph
Rate downstream of the boat
= (9.6 + 2.4) kmph
= 12 kmph
Time taken in covering 78 km distance
= $$\frac{{78}}{{12}}$$
= 6.5 hours
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