-3,5:4=x:20; -1,9:3=x:15; 4,1:-7=20,5:x;
-0.875=x:20
x=-0.875*20
x=-1705
0.633=x:15
x=0.633*15
x=9.495
-0.5857142857=20.5:x
x=-0.5857142857*20.5
x=-12.007142856