2*（18.5-3.15）+6.6/（0.75-0.2）
=2*15.35+6.6/0.55
=30.7+22
=52.7
0.15/2.1*56
=0.15/0.3*56/7
=0.5*8
=4
12.5/3.6-7/9+8.3/3.6
=（12.5-2.8+8.3）/3.6
=18/3.6
=5