12 species. There are two different coats, two pairs of trousers and three pairs of shoes. The wearing method is completed in three steps. There are three different options in the first step, two different options in the second step and three different options in the third step. According to the principle of multiplication, there is a method to get * * *: 2×2×3= 12 (species), which belongs to the problem of mathematical permutation and combination in primary schools.