Fourth, I do not get: we said, the events are independent, they all have the same probability to happen. So the probability of success at 3rd trial = probability o success of 5th trial, right? Now, this means the probability of success of 3rd AND 5th is equal to the probability of 4th and 5th, as well as 1st AND 2nd...and so on. This means we are not caring of which couples we are taking. This means probability of success of 2 general events, of a general couple over 5 trials must be equal to the probability of succeding EXACTLY at the 3rd and 5th thrial.
This is my reasoning. And I am afraid I am really not understanding.