GWD1-25

meckychen

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  How to calculate?

meckychen

up

sunny803

偶是手动排的，一个一个排出来。假设此高矮不同的六人从低到高，编号依次为 1 2 3 4 5 6。
由于每排从左到右及从前到后高度都是递增的，那个第一排最左边的一个一定是最矮的，就是1。而第二排最右边一个一定是最高的，就是6.

再具体来排，有以下几种排法：

1 2 3       1 2 4       1 2 5      1 3 4     1 3 5
4 5 6       3 5 6       3 4 6      2 5 6     2 4 6

共5种。
这个题目花了我3分半钟。用的土办法。不知道那个NN有更简单的方法解答此类题目。拜谢了