a=2b/3
c=1b/2
4b-2b/3a+2c=4b-2b/[3*(2b/3)]+2*(1b/2)=4b-1+b=5b-1
但是我估计楼主要问的应该是（4b-2b）/(3a+2c)=2b/[3*(2b/3)+2*(1b/2)]=2b/[2b+b]=2/3