试详答.

题是求第一次没过,第二次或第三次过的概率.

用G表过,M表没过.P(G)=87%,P(M)=1-87%=13%

P(M*G U M*M*G)= P(M*G)+P(M*M*G)+0 =

P(M)*P(G)+P(M)*P(M)*P(G)=13%*[87%+(1-87%)*87%]=0.127803