Ten Earthlings are abducted by aliens and lined up in single-file.
Each Earthling is given either a purple or green hat.
No one can see their own hat - they can only see the hats of the Earthlings in front of them.
Starting with the Earthling at the back of the line, each Earthling has to state correctly the color of her own hat, otherwise will be blasted by the aliens.
The Earthlings can only say “purple” or “green” (no intonations or saying it loudly or whatever).
The Earthlings are given ten Earth minutes to discuss a strategy.
How can at least nine Earthlings be saved?
The 10 Earthlings are P1 to P10 with P1 being at the back.
Assumptions - each Earthling can hear:
1. The others say their color.
2. The blast event - no blast means the correct color was said for survival. This assumption is significant for P3 to P10.
P1 looks ahead at the 9 Earthling in front of her and says purple/green to convey an even/odd number of greens in P2 to P10.
P2 says purple/green to make an even/odd number of greens in P2 to P10 as conveyed by P1. Ditto for P3 to P10.