1/3*(3/ 10*7/9+7/ 15*8/ 14+5/25*20/24)/( 1/3*(7/ 10+8/ 15+4/5))
=(7/30+4/ 15+ 1/6)/(7/ 10+8/ 15+4/5)
=(20/30)/(6 1/30)
=20/6 1
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