(1)0.8x:1.2=3:0.9,
0.8x×0.9=1.2×3,
0.72x=3.6,
0.72x÷0.72=3.6÷0.72,
x=5;
(2)5×70%+0.4x=9.1,
3.5+0.4x=9.1,
3.5+0.4x-3.5=9.1-3.5,
0.4x=5.6,
0.4x÷0.4=5.6÷0.4,
x=14;
(3)
3
4 x-
2
5 x=
7
10 ,
7
20 x=
7
10 ,
7
20 x÷
7
20 =
7
10 ÷
7
20 ,
x=2;
(4)
3
7 :
1
3 =x:
7
12 ,
1
3 x=
3
7 ×
7
12 ,
1
3 x÷
1
3 =
3
7 ×
7
12 ÷
1
3 ,
x=
3
7 ×
7
12 ×3,
x=
3
4 .