Analysis process:

18 = 2x9 = 6x3 = 9x2 =18x1or 9X2=3X6, a * * *, there are seven kinds of division methods.

The calculation method of permutation and combination is as follows:

The arrangement A(n, m)=n×(n- 1). (n-m+ 1)=n！ /(n-m)！ (n is subscript and m is superscript, the same below)

Combination C(n, m)=P(n, m)/P(m, m) =n! /m！ (n-m)！ ;

For example:

A(4，2)=4！ /2! =4*3= 12

C(4，2)=4！ /(2! *2! )=4*3/(2* 1)=6