Mathematical permutation and combination problems. How many kinds of seven people are divided into three, two, two and three groups? Important process.
Step 1, select 3 people from 7 people to form a group of C (7,3); Step 2, choose two people from the remaining four to form a group, and use C (4,2); Step 3: Make the remaining two people into a group. There are 1 methods. According to the multiplication formula, seven people can be divided into three groups * * C (7,3) * C (4,2) = 7 * 6 * 5/3! *4*3/2=2 10 species.