Piroska is Red Riding Hood, Nagymama is grandma on the picture:
The wolf attacked grandma, who lives on the coastline. As soon as Red Riding Hood heard Granny's scream, she went to help her. On what route should RRH move to save Granny within the shortest time? In the woods RRH runs with $v_1$ speed, and she swims with $v_2$ speed in the lake.
(The brown part is the woods and the blue is the lake. RRH can swim in a straight line since the lake won't drift her.)