If the last one is a boy's watch, then the probability that the first one is a girl's watch is:

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