Question

A boat takes 2 hrs to travel 8km down and 8km up a 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

Options:

2hr;3hr;2hr40min;3hr40min

Answer 1

In the first case, water is still, so the velocity of the boat in still water can be found out.
we know speed = distance / time
so speed= (8+8)/2 = 8km/hr
This is the speed of boat in still water
In the next case, the river is flowing with a velocity of 4km/hr.
​​​​​​So the resultant velocity of the boat downstream will be (8+4) = 12km/hr and the resultant velocity upstream will be (8-4) = 4km/hr.
so time taken is distance/velocity , ie
Time taken to travel downstream = 8/12 =2/3hr = 40min and
Time taken to travel upstream is 8/4 = 2hr .
so total time needed =
2hr 40 min