问题如下：
Assume :P(event A occurs)=15%, P(event B occurs)=35%, and P(event C occurs)=20%. If the event A, B and C are all independent, what is the P(event A or event C occurs)?
选项：
A.35%.
B.32%.
C.55%.
解释：
B is correct.
Event A, B and C are independent ,
so P(event A or event C occurs)=P(event A occurs)+ P(event C occurs)- P(both event A and event C occur)=15%+20%-15%*20%=32%
既然两件事情是独立的，为什么还会有两件事同时发生的情况呢？还要减去AC同时发生的概率？
