Source: CMU Puzzle Toad
Problem: Farmer Brown is standing in the middle of his perfectly circular field feeling very content. It is midnight and there is no moon and unknown to the farmer, Martian zoologists are landing randomly at points on the circumference of his field. They land at one minute intervals, starting at midnight. As soon as there are martians at points A,B,C such that triangle ABC contains the center of the field, Farmer Brown will be teleported to the waiting space-ship and transported to spend the rest of his life as an exhibit in a Martian zoo. What is the expected time until he is abducted?
Related Problem: http://pratikpoddarcse.blogspot.com/2009/10/semi-circle-covering-n-points-puzzle.html
Solution: Posted on CMU Puzzle Toad (http://www.cs.cmu.edu/puzzle/solution33.pdf). Check my name in the acknowledgments \m/ \m/