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%
老师，加法法则无论什么事件都可以使用吗？独立时间不是a的发生不影响b,为什么还要减去P(AC)?