UC知道关于概率，急！！
来源：百度知道 编辑：UC知道 时间：2024/05/31 08:27:13
Suppose that you have 20 different letters and 10 distinctly addressed envelopes. The
20 letters consist of 10 pairs, where each pair belongs inside one of the 10 envelopes.
Suppose that you place the 20 letters inside the 10 envelopes, two per envelope, but at
random.
What is the probability that exactly 3 of the 10 envelopes will contain both of the
letters which they should contain?
20 letters consist of 10 pairs, where each pair belongs inside one of the 10 envelopes.
Suppose that you place the 20 letters inside the 10 envelopes, two per envelope, but at
random.
What is the probability that exactly 3 of the 10 envelopes will contain both of the
letters which they should contain?
10组信,其中3组是正确的位置,有C3/10=120种方法
另外剩下的7组,14封信,有14!/2^7(次方)=681 080 400种方法
所以 概率P=120*681,080,400/(20!/2^10)=3.4399724 * 10 -5
即0.000034399724
算了好久给个最佳啊!~