3.29+0.73+2.27 =3.29+(0.73+2.27) =3.29+3 =6.29 (加法结合律) 3.29-0.73-2.27 =3.29-(0.73+2.27) =3.29-3 =0.29 (减法的性质) 7.5+2.5-7.5+2.5 =(7.5-7.5)+(2.5+2.5) =0+5 =5 (同级运算交换) 7.325-3.29-3.325 =7.325-3.325-3.29 =4-3.29 =0.71 (减法交换)