x/y=7/4, y/z=1/28.
4x=7y, y=4x/7; 28y=z, y=z/28. So, 4x/7=z/28, 4x=z/4, x=z/16, or z=16x, and z/x=16, z:x=16:1.
We can now write everything in terms of x: if x=1, y=4/7, z=16 which gives the ratio:
1:4/7:16. To get rid of the fraction in the ratio, multiply all the numbers by 7:
x:y:z=7:4:112. Now check: x:y=7:4; y:z=4:112=1:28.