一道关于概率的英文数学题

题目

一道关于概率的英文数学题
7) A box contains 3 red,4 white and 5 blue discs.Three discs are selected at random
from the box.Find the probability that:
a) all three discs are the same colour,if the selection is with replacement
b) all three discs are of different colours,if the selection is without replacement
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答案

额````翻译楼上的已经做了,我就直接解答了.a、颜色相同有3种情况,我们要分别计算就是1/4*2/11*1/10+1/3*3/11*1/5+5/12*4/11*3/10=3/44就是这个概率啦~b、这个跟上面挺像的,就是有点麻烦1/4*4/11*1/2+1/4*5/11*2/5+1/...