The formula of combination number refers to taking any m(m≤n) elements from n different elements to form a group, which is called the combination of taking m elements from n different elements; The number of all combinations of m(m≤n) elements from n different elements is called the number of combinations of m elements from n different elements. Represented by the symbol c(n, m). That is, above c, below m and below n.

c(n，m)=n！ /((n-m)！ *m！ )

c(n，m)=c(n，n-m)

C6(5)*C6(4) is C (6,5) * C (6,4).

C(6，5)*C(6，4)=C(6， 1)*C(6，2)=(6/ 1)*[(6 * 5)/2]= 6 * 15 = 90