(51+2/3)÷(5/3)+(71+3/4)÷(7/4)+(91+4/5)÷(9/5)=(50+5/3)*(3/5)+(70+7/4)*(4/7)+(90+9/5)*(5/9)=50*3/5+（5/3)(3/5)+70*4/7+(7/4)*(4/7)+90*5/9+(9/5)*(5/9)=30+1+40+1+50+1=123