You need to select 1 box and from that draw 1 object at random. What is the probability that the object drawn is black ?You need to select 1 box and from that draw 1 o
阅读背景：
给定三个盒子X, Y, z。让W表示白色的球，B表示黑色的球。盒子的内容是:X (2W, 3B)， Y(3W, 1B)， Z(1W, 4B)。
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