Its a pain to type so much word and code.
My question is on the right side of the page (XP 63a)
There is a question, and there are 3 ways of answering it (case a, b, and c)
(a), that is obvious.
(b) How do you get the first line? ( )
(c) Why does C equal to that? (Too lazy to type that much code, sorry)
Because there we have four pair-wise disjoint sets.
Consider any element of C. Then that element will belong to one of those four sets.
The importance of being pair-wise disjoint sets is that the probability of the union is simply the sum of the four probabilites.
If you know how to do Venn digrams, then it is easy to see.