你好!5/11*1/20 + 3/11*1/2 + 5/11*1/5= 5/11*(1/20+1/5) + 3/11*1/2= 5/11*1/4 + 3/11*1/2= 5/44 + 6/44= 11/44= 1/45/11*1/20 + 3/11*1/2 + 5/11*1/5= 1/44 + 3/22 + 1/11= 1/44 + 6/44 + 4/44= 11/44= 1/4