We don't need a whole lot of facts about probabilities in this course. The following will do:
Every probability is between 0 and 1, inclusive.
The sum of the probabilities of all possible events must be 1.0
If p is the probability that an event will happen, then (1-p) is the probability that the event will not happen.
If two events are independent and occur with probabilities p1 and p2, respectively, then the probability that both events will occur is p1*p2
If we make a number of independent attempts at something, and the chance of seeing an event on any single attempt is p, on average we will need 1/p attempts before seeing that event.