Question

有十名学生。女生三名，现在选两名代表，至少有一名女生当选的概率是？

Answer 1

反面全选男生的概率为C(2,7)/C(2,10)=7/15
∴P(至少有一名女生)=1-7/15=8/15

Answer 2

设男生为1,2,3,4,5,6,7女生为a,b,c用列举法有
(1,2)/(1,3)/(1,4)/(1,5)/(1,6)/(1,7)/(1,a)/(1,b)/(1,c)/(2.3)/(2,4)/(2,5)/(2,6)/(2,7)/(2,a)/(2,b)/(2,c)/(3,4)/(2,5)/(3,6)/(3,7)/(3,a)/(3,b)/(3,c)/(4,5)/(4,6)/(4,7)/(4,a)/(4,b)/(4,c)/(5,6)/(5,7)/(5,a)/(5,b)/(5.c)/(6,7)/(6,a)/(6,b)/(6,c)/(7,a)/(7,b)/(7.c)/(a,b)/(a.c)/(b,c)共有组合45种，其中有女生的有24种，所以至少有女生做代表的概率为24/45=8/15
请采纳~~