Oops. Didn't see the whole 3 weeks thing.

It has to do with "if" statements. I don't entirely remember how it goes, let's see if I can logic it back up.

If only one person had green eyes, they would see brown eyes on everyone else, hence would kill themselves that night, and the brown eyed people would kill themselves one day later.

If two people had green eyes, they would see the brown eyes on everyone else, except one other person. If that one other person didn't kill themselves that night, then each of the green-eyed people would know that they were the only green-eyed people. (The brown-eyed people wouldn't know what their eye color was yet, because they each saw two green-eyed people). The two green-eyed people would kill themselves on the second night, then the brown-eyed people on the third night.

If three people had green eyes, they each see two other people with green eyes and the rest with brown eyes (each of the brown-eyed people sees three people with green eyes). When, from the perspective of each of the people with green eyes, the other two green-eyed people don't kill themselves on the second night, then all three of the green-eyed people know that they are the three green-eyed people, and thus kill themselves on the third night. The remaining brown-eyed people kill themselves on the next night.

And repeat.

In this case, the number of green and brown-eyed people are the same, each of the people sees N other people with an eye color that isn't theirs, and N-1 people with an eye color that is theirs. After the (N-1)'th night, when the N-1 other people with shared eye color don't kill themselves, each person in the tribe realizes that they are the Nth person of their own eye color from their perspective. They all kill themselves on the Nth night, which is the 21st night in the riddle.

While Yarium had the right answer, I believe, 42 total tribefolk (killing themselves on the 21st night), it has nothing to do with the explorer being present, nor the explorer's own eye color.

They could have saved themselves from such a terrible fate by not counting the number of people with each eye color.

The same logic is usable on trappedslider's variant of the same puzzle.

If only one person had blue eyes, they would see everyone else with brown and leave the first night, with the brown-eyed people leaving the next night.

If two people had blue eyes, they would see one person with blue eyes not leave on the first night, everyone else would have brown, and so they would be the other person with blue. The two blue-eyed people would leave on the second night and everyone else on the third.

Since 100 people have blue eyes, they would all leave on the hundredth night, because each blue-eyed person would see 99 other people with blue eyes not leave on the 99th night.

However, in trappedslider's case, no one knows if the Guru saw someone with brown eyes, so each person with brown eyes was waiting to see if they were the 101-st person with blue eyes. They weren't, since all the blue-eyed people left on the 100th night.

Then also on the 100th night, each of the brown-eyed people knows that they see only brown-eyed other people (and the Guru, but she doesn't matter), but they don't know that each of the other brown-eyed people sees only brown-eyed other people.

But each of the brown-eyed people knows that each of the other brown-eyed people must see at least 98 other brown-eyed people. Therefore, each of the other brown-eyed people has implicitly stated "I see someone with brown eyes," which is a truism (each of the brown-eyed people that can be seen by any one brown-eyed person has seen someone with brown eyes by virtue of having seen 98 others with brown eyes). Since all of the brown-eyed people implicitly stated that on the first day, they also all leave on the 100th night. Being perfect logicians, they are able to use inductive, as well as deductive reasoning.

The Guru cannot deduce her own eye color, since no information about "green eyes" even existing has ever been given to her (no one implicitly or explicitly said they saw someone with "green eyes" and no one else actually has green eyes). She knows that she is the only one with her own eye color, but does not know what color it eyes (other than "not blue" and "not brown") The Guru remains behind for over 800 million more days until she can narrow down her own eye color from the approximately 24-bit color palette of the visible spectrum.

Aw. You're too kind.

Edit: Do those ***** translate into English? I can't figure out what they are supposed to mean. . .

x-1 people with green eyes might only see x-2 other people with green eyes, what can you expect from these x-1 people to be thinking? Just reduce each number in your previous question by one to receive your answer together with the next question. These mostly non-existing smaller groups of 2, 3, 4, ... people with green eyes couldn't be expected to know there's >1, >2, >3, ... people with green eyes until that's officially revealed on day 1, day 2, day 3, ... by that explorer or logical thinking. It's fascinating how that explorer's redundant information secretly affects this network of non-existing smaller groups of people that sends more and more signals to existing tribe members whenever nobody dies. Although there's an unspoken consensus between existing tribe members that plenty of people have green eyes, nothing happens until it's officially revealed that >1 people have green eyes.

The problem with this problem is that it presumes that all people think in exactly the same way and come to the same conclusions.

That rules out my first idea, where the explorer's confirmation of both brown and green eyes would lead at least one person to commit suicide. If you were the only person with brown/green eyes, saw that nobody else had them, and had confirmation from the explorer that both colours were present you would have to conclude that you were the only person with the brown/green color eyes. Following that logic, if you knew exactly how many other tribesmen had eyes of a particular colour you could use the number of days that nobody comes forward to figure out what you own eye colour was. Are there exactly 21 people of each eye colour on the island perhaps?
Yes, I realize I'm a bit behind the thread, but don't spoil it for me.