（2.9*2.5）*0.4
=2.9*（2.5*0.4）
=2.9*1
=2.9
.
7.65-（4.58-2.35）
=7.65-4.58+2.35
=7.65+2.35-4.58
=10 - 4.58
=5.42
.
30.5*20.8-305*1.08
=30.5*20.8-30.5*10.8
=30.5*（20.8-10.8）
=30.5*10
=305
.
（6.4-22.4/14）*0.35
=（6.4 - 11.2/7）*0.35
=（6.4 - 1.6）*0.35
=4.8*0.35
=2.4*0.7
=1.68