求助：GWD31-1-Q25 数学部分：哪位高手指点一下思路？

zhangkou · 发表于 2009-11-5 14:23:11

这道题我自己用弱智方法（一个一个排）算出来了答案，拜求各位高手指点简捷思路！！！！！
Q25:
A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row. The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her. How many such arrangements of the 6 people are possible?

A.   5
B.   6
C.   9
D.   24
E.   36
Answer: A

wangyuxin · 发表于 2009-11-5 16:18:21

这道题用的不是弱智解法，因为这种题目只有用列举法才能做出来有的时候一一列举未尝不是好方法。

taozike · 发表于 2009-11-5 17:18:18

先固定第一个数为1和最好一个数为6，然后第2个数只有两种可能，2，3，因为后面还有两个数比他大，按这样依次列举
1 2 3 4 5 6
1 2 4 3 5 6
1 2 5 3 4 6
1 3 4 2 5 6
1 3 5 2 4 6

求助：GWD31-1-Q25 数学部分：哪位高手指点一下思路？

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