(1-7/10)x=2.1/4 (1/4+1/5)x=1.4/5 (3.1/8+1.3/4)x=2.1/6 (2.1/12-1.5/6)x=3/4

1) x-7x/10=2.1/4;
10x/10-7x/10=21/40;
3x/10=21/40;
10*3x=21*10;
30x=210;
x=7;

2) (5/20-4/20)x=14/50;
x/20=14/20;
20x=20*14;
20x=280;
x=14;

3) (31/80+13/40)x=21/60;
(31/80+26/80)x=21/60;
57x/80=21/60;
57x*60=21*80;
57x*60=1680;
57x=28;
x=28/57.

4) (21/120-15/60)x=30/40;
(21/120-30/120)x=30/40;
-9x/120=30/40;
-360x=30*120;
-360x=3600;
x=-10.