0.25x401 =0.25x（400+1）=0.25x400+0.25x1=100+0.25 =100.25
0.25乘401=0.25乘400+0.25乘1=100+0.25=100.25
0.25x401=0.25x(400+1)=100+0.25=100.25
0.25x401 =25x4.01=25x4+25x0.01=100+0.25=100.25
