A man swimming in a river which flows 2.5 kmph finds that in a given time he can swim thrice as far with the stream as he can against it. How many distance he will cover in 8 hours against the current?

Let swimmer's speed in still water be x kmph. Downstream speed = x+2.5, upstream speed = x-2.5. Since distance with stream is 3 times distance against it in the same time: (x+2.5) = 3(x-2.5), giving x+2.5 = 3x-7.5, so 2x = 10 and x = 5 kmph. Against the current, speed = 5-2.5 = 2.5 kmph. In 8 hours, distance = 2.5×8 = 20 km.