A random variable is a function that assigns a numerical value to each simple event in a
sample space S.
Ex. Toss 3 coins. We are interested in the number of heads.
S= TTT, TTH, TH
In Section 8.3, we found the probability of A given B already happened - now we are going
to find the probability of an earlier event given the occurrence of a later event .
We will still use the formula:
P(AB) n(A B