A ratio is written like a:b (a to b), where a and b are your first and second numbers respectively. We can write the ratio for this problem to be the depth of the deepest part:shallow part, or 12:3.
A useful thing to know is that ratios can be looked at like fractions. This means that 12:3 could be visualized as 12/3. Simplified, we find that the final ratio is 4:1.
:)