To answer this, you need to divide the amount of one by the amount of the other. If we divide dry food by wet food, we have:
6.4 ÷ 2 = 3.2
12 ÷ 3.75 = 3.2
18 ÷ 5.625 = 3.2
20 ÷ 6.25 = 3.2
Since each combination is the same ratio, this is proportional.
What all of these tell you is that you need 3.2 times the amount of dry food than you use of the wet food. Or the dry:wet ratio is 3.2:1.