请教两道机经

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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?

请问这题如何入手呀? 谢谢了!

himba

用加法原理:

事件总数: C(1,100), 选PURPLE 的事件数: 20 + 15= 35

故 P = 35/100=7/20

或者: (1/2)(20/50) +(1/2)(15/50)= 7/20

请NN 指教.

mary1

片好此算法(1/2)(20/50) +(1/2)(15/50)= 7/20

最好选择盒子的概率知道