Two Chinese books are arranged in a (2,2). After finishing, we regard them as a big book and two math books as A (2,2). After arrangement, we also regard them as a big book, which means a big Chinese book, a big math book and a big physics book are arranged as A (3,3).

So the permutation and combination of the same theme are all adjacent: A (2,2) * A (2,2) * A (3,3) = 2 * 2 * 6 = 24.

The overall arrangement of this book is: a (5,5) =120.

So the probability of adjacent topics is: a (2,2) * a (2,2) * a (3,3)/a (5,5) = 24/120 =1/5.