[求助]GWD31套1-25

Sedulously

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

费费好象讲过,但是找不到了;一直不是很会排,除了全排出来没别的办法么?

~~~后天就考了,各位大大可能要多帮忙了~~

fernandochan

Alternatively, you can think about what can go in the first row. 1, 2, 3 are all safe, 4 and 5 are okay as long as they don't appear at the same time, and of course, 6 is out of the question.

That only leaves you choices of: {1, 2, 3}, {1, 2, 4}, {1, 2, 5}, {1, 3, 4}, {1, 3, 5}