(1)7.8x-10.8=4.2x,
7.8x-10.8+10.8=4.2x+10.8,
7.8x=4.2x+10.8,
7.8x-4.2x=4.2x+10.8-42x,
3.6x=10.8,
3.6x÷3.6=10.8÷3.6,
x=3;
(2)1.5(x+8)÷2=30,
1.5(x+8)÷2×2=30×2,
1.5(x+8)=60,
1.5(x+8)÷1.5=60÷1.5,
x+8=40,
x+8-8=40-8,
x=32;
(3)12.8-2.4x=4(x+2),
12.8-2.4x=4x+8,
12.8-2.4x+2.4x=4x+8+2.4x,
6.4x+8=12.8,
6.4x+8-8=12.8-8,
6.4x=4.8,
6.4x÷6.4=4.8÷6.4,
x=0.75.