4.587+3.47-2.47=4.587+(3.47-2.47)=4.587+1=5.587
先算3.47-2.47=1 再算第一项4.587+1=5.587
4.587+3.47-2.47=4.587+(3.47-2.47)=4.587+1=5.587
4.587+3.47-2.47=5.587