help prep2

bdsjb116 · 发表于 2010-9-20 11:44:49

tanya prepared 4 different letters to be sent to 4 different addresses. for each letter, she prepared an envelope with correct address. if the letters are to be put into 4 envelopes at random, what is the probability that only 1 letter will be put into the envelope with its correct address?
the answer is: 1/3
help me with it, please

wakjzmb · 发表于 2010-9-20 11:57:45

设有A,B,C,D四封信及相应的A*,B*,C*,D*信封。任选1个是正确的，方法为C(4,1);然后剩下的3个信要保证全部放错位置有2种放法，即C(2,1),SO，保证只放对1个信封的方法是C(4,1)*C(2,1). 然后除以总的放法：A(4,4). 8/24=1/3

bdsjb116 · 发表于 2010-9-20 12:07:58

got it!

thx

help prep2

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