Diseases and blue marks

Score Points: 5Solvers: 12

45 Natives in a certain colony have a myth that there is a certain disease signalled by a blue mark on their forehead. All the natives have blue marks. If they find out that they have the disease, they must declare it the same day in the town hall meeting, and leave the colony. However, no one else tells others if they have a blue mark out of courtesy. One day, a foreigner comes in, and tells the whole society that he has seen at least one blue mark in the colony. If the day the foreigner came was day 1, on what day will the first person leave the village? If you think they will never leave, enter 0.

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