Beach Walking

Anne and her friend are walking on the beach. At least, her friend is walking. Anne runs ahead at 3 times his speed. When she reaches the endpoint, 4 kilometers north of the starting point, she turns around and runs back at the same speed. When she meets her friend, she turns around again. And this goes on endlessly.

They also have a dog with them. And it is unbelievable, but the dog runs twice as fast as Anne, endlessly back and forth between Anne and her friend. Eventually, all three arrive together at the endpoint.