[理工] 機率

[KAINTS (RUKAWA)]

1.
A ordinary deck of 52 cards is dealt,13 each,at random
among A,B,C,and D.Waht is the probability that A and
B together get two aces?

我一開始是算4C1*48C12*3C1*36C12/52C13*39C13
但解答給的是4C2*48C24/52C26

為什麼他是一起考慮???是因為"together"這單子嗎??
A and B一起取這樣??

2.
From the set of all families with two children,(a)a family is selected
at ranndom and is found to have a girl,what is the prob. that the
other child of the family is a girl?,(b) a child is selected at random
and is found to be a girl,what is the prob. that the second child of
this girl's family is also a girl? Assume that in a two-child family
all sex distributions are equally probale.
答案分別是(a) 1/3,(b) 1/2
想請問一下這兩題問的差異在哪裡?

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※ 編輯: KAINTS          來自: 111.70.247.44        (10/16 15:57)
→ shcyril:2(a)是條件機率 在至少一女的條件下 另一個也是女的機率
→ shcyril:女女 / (男女+女男+女女) = 1/3
→ shcyril:2(b)的條件不影響機率 故1/2 這題在考英文XD
→ KAINTS:可是我看閃電本還是使用條件機率解ㄟ，後來我想想是跟famil
→ KAINTS:y & clild 有關吧，第一題要從有女生的家庭去挑一個也是女
→ KAINTS:的機率。而第二題則是問我小孩是女生而另一個也是女生的機
→ KAINTS:率，所以(1/4)/(1/2)
→ KAINTS:不知道這樣想對嗎