[求助]GWD10-25

yaskemail

Q25.

A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of thress 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

Any easy way to find out the answer? Thanks!

smart

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

yaskemail

thanks!