Can 3 oz and 30 inches be expressed as a ratio?
The answer is Yes.
Expressing it into a ratio will be 3:30 or 1:10 in simplified form.
When is it that it cannot be expressed as a ratio?
A given number or value cannot be expressed as a ratio when it is an irrational number or a number that cannot be written in simplified form.
Example of irrational numbers: Square Roots/Radicals (√8,.. etc.), π, and other numbers with infinite decimal value and cannot be transformed into fractions (4.54353..., etc.)