(15-4.5)/0.5=10.5/0.5=21
(15-4.5)÷0.5
=15÷0.5-4.5÷0.5
=30-9
=213.67×0.4﹢4×0.633（用简便计算）!急急急！快快6.08÷0.19﹣4.2×5(用简便计算）！急急急！快快