在OG的math review里，讲到"Discrete probability"，请指点：

"Two events A and B are said to be independent if the occurrence of either event does not alter the probabiity than that the other event occurs. For one roll of the number cube, let A={2,4,6}, and let B={5,6}。Then the probability that A occurs is P(A)=3/6=1/2"

粗体的那句话不懂，然后不明白为什么除以6啊，A里面有三种可能这个我理解，可是6是从哪里来的呢？

多谢各位了.