9.9*45+4.5=99*4.5+4.5*1=4.5*(99+1)=4.5*100=450
2007*(2006/2005)=2007*(1+1/2005)=2007+2007/2005=2008又2/2005
(369/20)/9=(369/9)/20=41/20
9900/25=99*100/25=100/25*99=4*99=4*(100-1)=400-4=396