3.85×99+3.8=3.85x99+3.85-0.05=3.85x（99+1）-0.05=3.85x100-0.05=385-0.05=384.95
3.85x99+3.8＝3.85x（100-1）+3.8＝3.85x100-3.85x1+3.8＝385+3.8-3.85＝385-0.05＝384.95
