In the 4× 100-meter relay race of the school track and field sports meeting, the runway is arranged by lottery, and the ninth grade (1), (2) and (3) classes are just divided in.

∵ There are six possible results, and the possibility of each result is equal. Among them, (1) and (2) just rank first and second in turn, with only 1 result.

∴p( 1 class and class 2 just rank first and second in turn) =