14.56-2.99
=14.56-3+0.01
=11.56+0.01
=11.57
13.45-3.19-2.81
=13.45-(3.19+2.81)
=13.45-6
=7.45
（15.2-9.9 ）-（4.3-2.540 ）
=15.2-10+0.1-(4.3-2.54)
=5.2+0.1-4.3+2.54
=5.3-4.3+2.54
=1+2.54
=3.54