（80-9.8）*0.6-2.12
=[80-(10-0.2)]*0.6-2.12
=[80-10+0.2]*0.6-2.12
=[70+0.2]*0.6-2.12
=70*0.6+0.2*0.6-2.12
=42+0.12-2-0.12
=40
3.47*99+3.47*10
=3.47*(100-1)+3.47*10
=3.47*100-3.47*1+3.47*10
=347-3.47+34.7
=378.23
（1/4+3/8）÷（1-3/4*2/5）
=(2/8+3/8)÷（1-3/10）
=(5/8)÷（7/10）
=5/8*10/7
=50/56
=25/28
（2/15+1/12）*30-1.25
=（2/15+1/12）*30-5/4
=2/15*30-1/12*30-5/4
=4-5/2-5/4
=4-10/4-5/4
=4-15/4
=16/4-15/4
=1/4