回复：P(2)=18/35,P(0)=0,P(1)=4/35,P(3)=12/35,P(4)=1/35.

P(0) = C(4,0)*C(3,4)/C(7,4) = 1*0/35 = 0/35

P(1) = C(4,1)*C(3,3)/C(7,4) = 4*1/35 = 4/35

P(2) = C(4,2)*C(3,2)/C(7,4) = 6*3/35 = 18/35

P(3) = C(4,3)*C(3,1)/C(7,4) = 4*3/35 = 12/35

P(4) = C(4,4)*C(3,0)/C(7,4) = 1*1/35 = 1/35

P(4)+P(1)+P(2)+P(3)+P(4)

= (0+4+18+12+1)/35

= 1

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