prep practice 1中一些数学和语法题，没有在破解的材料中找到解法，希望各位帮忙解释一下。谢谢！

zhangjy

希望大家能帮我看看这些题该怎么解。都是我在破解材料中没有找到答案的。

seyouji

数学：
首先一共有4×3×2×1=24种可能；
ABCD四个信封，abcd四封信；
a放入A时：acdb 和 adbc两种情况
所以共有2×4种
结论=8/24

语法题，用google搜吧，Manhattan SC给的答案我比牛。

zhangjy

希望大家解读一下啊。。谢谢！！！！

ycivan

Tanya prepared four different letters to be sent to four different addresses. For each letter, she prepared an envelope with its correct address. If the 4 letters are to be put into the envelopes at random, what is the probability that only one letter will be put into the envelope with its correct address?

Answer:

(C4,1*C2,1)/P44 = 1/3

C4,1: firstly, there are for chances for a letter is put into the correct envelope.
C2,1: secondly, there are two chances for a letter is put into the incorrect envelope.
Then, there is only one choice to put a letter into the incorrect envelope in the third and forth time.