Counting Rules

P 係排列意思囉
n 係 total 共幾多數咩
x 係取幾多數咩
! 係階乘
for e.g
n for 6, x for 2
it can be write down as
6P2 = 6! / ( 6 - 2 )! = 6! / 4! = 6 * 5 * 4! / 4! = 6 * 5 = 30
P.S 6! = 6 * 5 * 4 * 3 * 2 * 1