A boat can tarvel 36 km upstream in 5 hours. If the speed of the stream is 2.4 kmph, how much time will the boat take to cover a distance of 78 km downstream?(in hours)

Answer: Option B

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