请教一概率题（最新机经的说）

lqlucas

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！

terry_tin

我觉得答案应该如下：
1/2*20/50+1/2*15/50=35/100=7/20

joywzy

agree with TT.

lqlucas

Thank you TT!
I misunderstood the meaning of the question.