@fantasticcrow That's a really great question!

Listing out all the "exit possibilities" could definitely work-- but you have to be careful, because not all the possibilities are equally likely. For example, it's a lot more likely to go from the start point, to the top left room, and then out through E (probability 1/3) than it is to go from the start point, down, down, left, and out through C (probability 1/12, as calculated in the solution). This is mainly because the second path is "longer."

So basically, when you list out all possible ways to exit, **not all** the paths are going to be equally likely to occur. That's why you can't just count the number of paths out of A, B, and C and divide by the number of paths out from anywhere. (To put it a different way, we can't treat it like the probability of choosing a green marble if we have 5 green marbles and 7 total marbles-- the marbles are different sizes, which implies that some are more likely to be chosen than others)

Hope this helped! 😄