1) 4/a=16/20; 2) c/8=25/40; 3)39/27=b/9; 4)49/32=12/d;

1.
   4/a = 16/20
   a = 4 * 20 : 16 = 5
2.
   с/8 = 25/40
   с = 8 * 25 : 40 = 5
3.
   39/27 =  b/9
   b = 39*9 : 27 = 13
4.
   49/32 = 12/d
   d = 32 * 12 : 49 = 384/49