Question:

A world-class swimmer can swim at twice the speed of the prevailing tide. She swims out to a buoy and back again, taking four minutes to make the round-trip. How long would it take her to make the identical swim in still water?

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.

C

A

P

T

A

I

N

I

N

T

E

R

V

I

E

W

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Solution: We know that she swims at twice the speed of the tide. That means that when swimming against the tide, her speed is equal to that of the prevailing tide. In still water, then, she swims at twice the speed of the tide. If we call the time it takes to swim the distance between the shore and buoy 1 splash, then with the tide it will take one-third splash and in still water one-half splash.

To swim there and back, then, takes 1 splash in still water and four-thirds splash when there is a tide (since it takes 1 splash for the part of the swim that is against the tide and one-third splash for the part of the swim that is with the tide). This is one-third as long again as the time taken to do the round-trip in still water. Since it takes four minutes when there is a tide, it must take three minutes when there is no tide.

Solution: Three minutes.