bigfernhead wrote:

A swimmer makes a round trip up and down the river. It takes him X hours. The next day he swims the same distance with the same speed in the still water. It takes him Y hours. What can we say about X and Y ?

X>Y

X<Y

X=Y

X=1/2Y

none of the above

Hi,

x: speed of the swimmer (km/h)

y: speed of the water (km/h)

So:

x-y: speed of the swimmer down the river

x+y: speed of the swimmer up the river

Because the distance doesn't have an important role in this problem so I will assume that the distance is 1km

(this swimmer is so strong)

=> Time of the trip at the first day:

1/(x-y) + 1/(x+y)= 2x/(x2-y2)

time of the trip at the second day:

2/x= 2x/x2

We can easily see that

2x/(x2-y2) > 2x/x2

=> X> Y

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