2又3分之一-12分之7+30分之11+42分之13-56分之15+72分之17-90分之19

7/3-7/12+11/30+13/42-15/56+17/72-19/90
=7/3-(1/3+1/4)+(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+(1/8+1/9)-(1/9+1/10)
=7/3-1/3-1/4+1/5+1/6+1/6+1/7-1/7-1/8+1/8+1/9-1/9+1/10
=7/3-1/3-1/4+1/5+1/6+1/6+1/10
=2-1/20+1/3+2/20
=2+1/20+1/3
=2+3/60+20/60
=2+23/60
2又23/60