Question

a boat takes 2hrs to travel 8km and back in still water lake.with water velocity of 4kmph,the time taken for going upstream of 8km and coming back is

Answer 1

Distance = Velocity * Time

8 + 8 = V * 2
16 = 2V
V = 8

So the boat travels at 8 km/hr

If the river starts flowing at 4 km/hr, then we get 2 formulas:

8 = (8 - 4) * t[1]
8 = (8 + 4) * t[2]

We want to add t[1] and t[2] to get the total time T

T = 8 / (8 - 4) + 8 / (8 + 4)
T = 8/4 + 8/12
T = 2 + 2/3
T = 8/3
T = 2 hours 40 minutes