(1)“飞碟投入红袋”,“飞碟投入蓝袋”,“飞碟不入袋”分别记为事件 A , B , C . 则 P ( A )= = , P ( B )= P ( C )= = . 因每次投掷飞碟为相互独立事件,故4次投掷中恰有三次投入红袋的概率为 P 4 (3)= 3 = . (2)两次投掷得分 ξ 的得分可取值为0,1,2,3,4则: P ( ξ =0)= P ( C ) P ( C )= ; P ( ξ =1)= P ( B ) P ( C )=2× × = ; P ( ξ =2)= P ( A ) P ( C )+ P ( B ) P ( B )= ; P ( ξ =3)= P ( A ) P ( B )= ; P ( ξ =4)= P ( A ) P ( A )= . ∴ E ( ξ )=0× +1× +2× +3× +4× = .