Counting - Permutations & Combinations Two series of a question booklet for an aptitude test are to be given to twelve students. In how many ways can the students be placed in two rows of six each so that there should be no identical series side by side and that the students sitting one behind the other should have the same series?

Now you have to divide the 12 students in two series each of them have 6 student .
So No. of ways that the students be placed  is $12\cdot11\cdot 10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1$  
Since  there should be no identical series side by side and that the students sitting one behind the other should have the same series , each series must have the same booklet. $\implies$ No. of ways to place the booklet equals $2$
$X:1,2,1,2,1,2$ & $Y:1,2,1,2,1,2$  OR $X:2,1,2,1,2,1$ &  $Y:2,1,2,1,2,1$
Therefore the total ways to place the student with the  conditions mentioned in the question is  $2\cdot12\cdot11\cdot 10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1$ 
I'm NOT sure that my solution is true !
