80-12×4÷3 = 80 - 4×（12÷3）= 80 - 4×4 = 80 - 16 = 64
60+180÷20-5 = 60 + 9 - 5 = 64
4.5-（2.68+0.86）=4.5 -（2.7-0.02+0.8+0.06）=4.5 -（3.5+0.04）= 4.5 - 3.5 - 0.04 =0.96
15.37-2.8-2.2 = 15.37 - （2.8+2.2）=15.37 - 5 = 10.37
19.92+10.35-9.92 =10.35 + （19.92 - 9.92）=10.35 + 10 = 20.35
42×4×25 = 42×（4×25）= 42 ×100 = 4200