The Fox and The Duck

Question:

A duck that is being chased by a fox saves itself by sitting at the center of the circular pond of radius r. The duck can fly from land but cannot fly from the water. Furthermore, the fox cannot swim. The fox is four times faster than the duck. Assuming that the duck and fox are perfectly smart, is it possible for the duck to ever reach the edge of the pond and fly away to its escape from the ground?

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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: One would think that the duck could swim directly away from the fox. So the duck would have to swim a distance r. The fox would have to cover half the circumference of the pond (pi * r). Since the fox is four times faster than the duck, we know that

(pi * r) < (4 * r)

This would make it seem like it is impossible for the duck to escape.

So how could the duck make life most difficult for the fox? If the duck just tries to swim along a radius, the fox could just sit along that radius and the duck would continue to be trapped.

The duck could swim in concentric circles so that the fox has to continuously run along the circumference of the pond to stay on the same radius as the duck. If the duck swims near the edge of the pond, the fox could easily keep up since they would be covering approximately the same distance and the fox is four times faster. But what if the duck swam closer to the center of the pond? The duck would have to cover a smaller circumference and could use this strategy to put some distance between the fox and itself. At a distance of r/4 from the center of the pond, the circumference of the pond is exactly four times the circumference of the duck’s path. Thus, to stay on the same radius as the duck, the fox would barely keep up.

Say, the duck circles the pond at a distance r/4 – e, where e is an infinitesimal amount. So as the duck continues to swim along this radius, it would slowly gain some distance over the fox. Once the duck is able to gain 180 degrees over the fox, the duck would have to cover a distance of 3r/4 + e to reach the edge of the pond. In the meanwhile, the fox would have to cover half the circumference of the pond (i.e the 180 degrees). At that point,

(pi * r ) > 4 * (3r/4 + e)

The duck would be able to make it to land and fly away.

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