GMAT Quantitative Reasoning Practice Test 2

The leftmost book is a mathematics book; the next is physics and so on. So, the odd places books are the mathematics books and the even placed books are the physics books. The mission is therefore, arranging three mathematics books in three places $(3!=6$ arrangements $)$ and three physics books in three places $(3!=6$ arrangements).The total number of different arrangements is the product of the two missions: $3!\times 3!=6\times 6=36$.