51又3分之2×5分之3＋71又4分之3×7分之4＋91又5分之4×9分之5
=(50+5/3)x3/5+(70+7/4)x4/7+(90+9/5)x5/9
=50x3/5+5/3x3/5+70x4/7+7/4x4/7+90x5/9+9/5x5/9
=30+1+40+1+50+1
=123
如果本题有什么不明白可以追问,