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①5x+300=650,
5x+300-300=650-300,
5x=350,
5x÷5=350÷5,
x=70;
②2.8x-2x=4,
x(2.8-2)=4,
0.8x=4,
0.8x÷0.8=4÷0.8,
x=5;
③4x-3×18=104,
4x-54=104,
4x-54+54=104+54,
4x=158,
4x÷4=158÷4,
x=39.5;
④
:x=1 4
:1 8
,1 10
x=1 8
×1 4
,1 10
x=1 8
,1 40
x÷1 8
=1 8
÷1 40
,1 8
x=
.1 5