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Assume you are not a troll, the max chance of troll showing up is 4/(10-1) = 44.4% for the current team, and the chance of becoming a captain is 1/5 = 20%, the chance of captain is a troll is about 8.88%

you cannot know what the chance of a troll showing up is unless you know the statistics on how many trolls to good players there are. However, the MAX chance of a troll showing up would occur in a situation where you were the only good player and every other single player was a troll. If this were true, there would be a 100% chance of 4 trolls on your team and an 80% chance a troll was captain.

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We can do this way, what is the max number of people in the game that can be a troll and that is also on your team. (since people on the other team can't be your captain), that is:

(4/9)

then what is the chance that a player on your team can be a captain?

(1/5)

the chance of having a troll become captain has nothing to do with the total of 10 people in the game. It has purely to do with the 5 people on your team (or the 4 other people if you assume you are not a troll). the max number of people that are "in your game" is unimportant. the question should be:

what is the maximum number of people on your team that can be a troll?

and the answer is:

4