Solution

Although we neither know the exact velocity nor the exact time, we can help the huntsman: The huntsman (blue) covers the distance s, therefore it is:

But the dog covers another distance during the same time:

that means that the distance which the dog covers (red) only depends on his velocity. If the dog runs twice as fast as the huntsman, he will cover twice the distance. The same applies to three times, four times...etc. the velocity. It will be easier, if you look at the velocity-time-diagram of both of them:

Due to the fact that the area below the straight line always represents the covered distance, you see that the dog (yellow rectangle) always covers three times the distance of the huntsman, if he runs at three times the velocity of the huntsman (blue rectangle). For example, after one hour:

If you do not believe it, you can count the small boxes within the yellow and the blue rectangle...