challenging question

rogersun

You have a bag of 9 letters: 3 Xs, 3 Ys, and 3 Zs. You are given a box divided into 3 rows and 3 columns for a total of 9 areas. How many different ways can you place one letter into each area such that there are no rows or columns with 2 or more of the same letter? (Note: One such way is shown below.)


(A)    5
(B)    6
(C)    9
(D)   12
(E)    18

an example as following
X Y Z
Y Z X
Z X Y

anchoret

固定第1列，第2列的变化只有2种；
当前两列确定时，第3列已经被确定；
所以：p(3,3)*2=12，选D