They do NOT know about the 100:100 ratio (only we do). Furthermore, they do not know that the only eye colors present are blue and brown. Each one must consider the possibility that he/she has an arbitrary color (e.g. green).
ah! ok. If that's the case, I can only see one possibility: using water as a mirror! Ahhah
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Think about the case of just one person having blue eyes. What would they know and do?
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