1999.9+199.9+19.9+1.9 =(2000-0.1)+(200-0.1)+(20-0.1)+(2-0.1) =2000+200+20+2-0.1×4=2222-0.4=2221.6
1999.9+199.9+19.9+1.9=2000-0.1+200-0.1+20-0.1+2-0.1=2222-0.1*5=2221.5
