2.5*9.9=2.5*(10-0.1)=2.5*10-2.5*0.1=25-0.25=24.75.
2.5x（10-0.1）=2.5x10-2.5x0.1=25-0.25=24.75
=2.5×(10-0.1)=2.5×10-2.5×0.1=25-0.25=24.75
