15.6+(64.4-17.8)=(15.6+64.4)-17.8=70-17-0.8=53-0.8=52.2
把括号去掉再进行计算。即15.6+(64.4-17.8)=15.6+64.4-17.8=80-17.8=62.2
=(15.6+64.4)-17.8
=15+0.6+64+0.4 -17-0.8=15+54-17=54-2=52