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+1+40+1+50+1
=123