l两道Preceton Math

cathyyan

向牛牛求解题过程：

1.A part of game. foure people must each secretly select an integer between 1 and 4, inclsive, what is the approximat likehood that all four different pepole choose differenct number.
a) 9%
b)12%
c)16%
d)20%
e)25%
answer: a


2) out of 7 models, all of different hights, 5 models will be to chose to pose a photograph, if the 5 modes will be stand in a line from shortest to tallest,and the 4the tallest and 6 talles can not be adjacent, how many of 5 arrangement are possible.
a)6
b)11
c)17
d)72
c)210
answer: c

mymengming

以下是引用cathyyan在2008-5-25 20:24:00的发言：
向牛牛求解题过程：

1.A part of game. foure people must each secretly select an integer between 1 and 4, inclsive, what is the approximat likehood that all four different pepole choose differenct number.
a) 9%
b)12%
c)16%
d)20%
e)25%
answer: a

第一个人可以任意抽，第二个人从剩下的三个中抽，……

所以是4/4×3、4×2/4×1/4

2) out of 7 models, all of different hights, 5 models will be to chose to pose a photograph, if the 5 modes will be stand in a line from shortest to tallest,and the 4the tallest and 6 talles can not be adjacent, how many of 5 arrangement are possible.
a)6
b)11
c)17
d)72
c)210
answer: c

把model想成1，2，……7个数

首先从7个中选5个出来C5,7

然后还要减掉选出4和6，但是没有选5的时候，这时候是C3,4

C5,7-C3,4=17