[計量] 一題機率問題請教
There are 3 blue disks, 5 green disks, and nothing else in a container. Disks
will be chosen one at a time, and at random and without replacement, from the
container, until all 3 blue disks have been chosen. What is the probability
that the third blue disk will be the seventh disk chosen?
(A) 3/28
(B) 1/8
(C) 5/28
(D) 15/56
(E) 3/8
答案(D)
我的想法是: 前面六次不管怎麼取都沒關係，只要第七次是紅色就好
3 2 5 4 3 2
C1*C1*C1*C1*C1*C1*1
_____________________
8 7 6 5 4 3 2
C1*C1*C1*C1*C1*C1*C1
可是答案似乎不是這樣算 @@"
請教各位了~謝謝!!
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