①5.4-X=1.2,
5.4-X+X=1.2+X,
1.2+X=5.4,
1.2+X-1.2=5.4-1.2,
X=4.2;
②0.6X=42,
0.6X÷0.6=42÷0.6,
X=70;
③X÷2.4=0.5,
X÷2.4×2.4=0.5×2.4,
X=1.2.
①5.4-X=1.2,
5.4-X+X=1.2+X,
1.2+X=5.4,
1.2+X-1.2=5.4-1.2,
X=4.2;
②0.6X=42,
0.6X÷0.6=42÷0.6,
X=70;
③X÷2.4=0.5,
X÷2.4×2.4=0.5×2.4,
X=1.2.