①4x+4.8=20
②5(x+0.4)=5
③3(x-2.4)÷5=1.8
④0.3×2.8+1.5x=3.6x
1- = + = ×2= + = - =
18× = ÷6= ÷ = a-0.4a= + + + =
= = =
= =
前两小题中的单位“1”分别是什么?