3.29+0.73+2.27 =3.29+(0.73+2.27) =3.29+3 =6.29 (加法结合律) 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-(5.325+1.7) =7.325-5.325-1.7 =2-1.7 =0.3 (减法的性质)