紧急求助prep software 的数学解答思路

jillgaocn · 发表于 2009-12-10 11:57:49

does anyone has the discussion about to how to solve some of the maths problems? I have to sit for it next Monday. Help. Thanks so much in advance
If there is no discussion, i have a question i am a little confused
Tanya prepared 4 different letters to be sent to different address. For each letter, she prepares envelop with correct address. if the 4 letter are to be put into 4 envelops at rando, what is probability that only one letter will be put into the envelop with the correct address? the answer is 1/3

genius_fang · 发表于 2009-12-10 13:00:26

首先选1封放进正确的信封，有4种选法，然后余下来3封要全部打乱，那么其实只要确定这3封里面的一封放进错误的信封，余下来的两封只有一种放法了。所以3封全部打乱的放法只有2种（即对于第一封他有两个错误的信封给他选，一旦他选定了，余下来两封的位置也就定下来了）。所以放对一封信得的排列方法有：4*2=8种，这是分子，然后，就是所有放的方法：P4，结果就是8/P4=1/3

jillgaocn · 发表于 2009-12-10 13:17:45

many thanks!

紧急求助prep software 的数学解答思路

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