51又2/3*3/5+71又3/4*4/7+91又4/5*5/9
=（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+40+50+1*3
=123