A boat takes a total time of 32 hours to travel 120 km downstream and the same distance upstream. If the speed of the boat in still water is four times that of the current, what is the time taken by the boat to travel 80 km downstream?

12 hours
2 hours
8 hours
5 hours

Correct answer: 8 hours

Solution

If boat speed in still water is 4x and current speed is x, then downstream speed = 5x and upstream speed = 3x. Given 120/5x + 120/3x = 32, solving gives x = 2 km/h, so downstream speed = 10 km/h. Time for 80 km downstream = 80/10 = 8 hours.

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