In box A, there are 20 purple 30 red marbles. In box B, there are 15 purple and 35 red ones. If someone randomly select the box first then select one marble from the box, what is the posiblity to select a purple one? 机经答案:9/20 我的答案:(20/50*35/50)+(30/50*15/50)=23/50 欢迎大家讨论!