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A boat takes 2 hours to travel 8 km and back in still water lake. With water velocity of $4 km h^{-1}$, the time taken for going upstream of 8 km and coming back is
Explanation
Velocity of boat = $(8+8)/2 = 8kmh^{-1}$ Velocity of water = $4kmh^{-1}$ $t = 8/(8-4) + 8/(8+4) = 8/3 h = 160$ minute