At first I thought there was a bug in experiment 2b he writes that the sample space should be:<p>{'BB/?b', 'BB/b?', 'BG/b?', 'GB/?b'}<p>Because he describes the event as "He is observed at a time when he is accompanied by one of his children, chosen at random."<p>I thought he also needed to include two more cases:<p>{'BB/?b', 'BB/b?', 'BG/b?', 'GB/?b', 'GB/g?', 'BG/?g'}<p>Which again gives us 1/3 probability of both being boys.<p>but I guess the part that comes after the '/' indicates the observation event.