Final answer:
The question is a logical deduction based on the concept of subsets, where Portland is a subset of Oregon, leading to the valid conclusion that if Cara doesn't live in Oregon, she can't live in Portland.
Step-by-step explanation:
The question at hand involves logical reasoning and can be associated with the concept of subsets in mathematics. The statement 'If you live in Portland, you live in Oregon' establishes that Portland is part of Oregon, or in set-theory terms, Portland is a subset of Oregon. So, if Cara does not live in Oregon, then it is logically impossible for her to live in Portland since Portland is contained within Oregon. Therefore, using the concept of subsets, the conclusion 'Cara does not live in Portland' is valid. This type of reasoning is often associated with Venn diagrams and logical deductions.