Question

A boat takes two hours to travel 8km down and 8km up the river when the water is still how much time will the boat take to make the same trip when the river starts flowing at 4km/hr??

Answer 1

Speed of boat = 16/2 = 8kmph.
Now when riverflows.

Downstream : - Distance = 8km
Speed = 8 + 4 = 12
Time = 8/12 = 2/3 hout

Upstream : - D= 8km
Speed = 8-4 = 4
Time = 8/4 = 2hr

Answer 2

Answer:

So it can go 8 km up in 1 hour and 8 km down in 1 hour.                                    So velocity of boat in still water is 8km/hr

Vbr = velocity of boat wrt to river= 8 km/hr

Vrg = velocity of river wrt to ground= 4 km/hr

Downstream velocity of boat = 8+4 =12 km/hr

Upstream velocity of boat = 8−4 = 4 km/hr

Time taken = 8/12 + 8/4 = 2/3  +2 = 2.6666 km/hr.

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