（3/5-3/4*5/3)/1/5 =（3/5-3/4*5/3)*5 =(3/5-5/4)*5 =3-25/4 =12/4-25/4 =-13/4 =-3又4分之18/7*(21/8-7/4)/2/3 =(8/7*21/8-8/7*7/4)*3/2 =(3-2)*3/2 =3/2 =1又2分之1