[rot 13, I don't know if HN has spoiler tags]
Nffvta n qvssrerag inyhr zbq A rnpu naq rnpu thrff gur ahzore gung jbhyq znxr gur gbgny gung inyhr vs vg jnf ba lbhe urnq?
Nffvta n qvssrerag inyhr zbq A rnpu naq rnpu thrff gur ahzore gung jbhyq znxr gur gbgny gung inyhr vs vg jnf ba lbhe urnq?
I assumed the sequence is (in effect):
- Quietly look at the numbers on everyone else
- Quietly decide what number to shout
- All shout when a signal is given
- Somehow hear and understand what everyone shouted simultaneously
- Quietly decide on your guess (at the number on your own forehead is)
- All reveal their guesses when a signal is given
If that's correct - then everyone can just shout the lowest number they see on someone else, then give the lowest number they heard shouted as their guess. That number is on someone's forehead, and one correct guess lets everyone go free.