一等A,二等B,三等C(A+B)/(A+B+C)=0.4(B+C)/(A+B+C)=0.9得到 A/(A+B+C)=0.1 C/(A+B+C)=0.6 故 B/(A+B+C)=0.3
2/5+9/10-1=3/10
等级3/10！谢谢刘揽谢谢
