记Bi=5个里面有i个品质好的,i=0,1,2,3,4,5
AI=在Bi下,2个里面没有一个品质好的
P(B0)=C11,5/C30,5=11/3393 P(A0)=1
P(B1)=C11,4×C19,1/C30,5=1045/23751 P(A1)=C4,2÷C5,2=3/5
P(B2)=C11,3×C19,2/C30,5=1045/5278 P(A2)=C3,2÷C5,2=3/10
P(B3)=C11,2×C19,3/C30,5=17765/47502 P(A3)=C2,2÷C5,2=1/10
P(B4)=C11,1×C19,4/C30,5=7106/23751 P(A4)=0
P(B5)=C19,5/C30,5=646/7917 P(A5)=0
P=1－P(B0)P(A0)－P(B1)P(A1)－P(B2)P(A2)－P(B3)P(A3)－0－0
＝1－11/3393－1045/23751×3/5－1045/5278×3/10－17765/47502×1/10
≈0.87
所求概率为0.87