The formula of permutation and combination c: C(n, m)=A(n, m)/m! .

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。

A32 is permutation, C32 is combination.

For example, A32 is 3 times 2 equals 6.

A63 is 6*5*4.

That is, starting from a big number and multiplying by a later number indicates how many numbers there are. A72 equals 7*6*2, so there are two numbers A52=5*4.

Then C32 is divided by a number, for example, C32 is A32 and then divided by A22.

C53 is A53 divided by A33.