-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