The question is very simple and I think the professor intended to test if we understand $P(A\cup B)\geq P(A)$. Below is the full question:
Lance is 29. He attended Michigan State University and majored in business. He obtained a 2.8 GPA. Now he runs a small company doing web-based marketing and spends the rest of his time watching ESPN, playing video games, and cruising bars.
Consider the following statements about Lance:
S1. Lance partied a lot in college
S2. Lance really likes reading Jane Austen
S3. Lance likes barbeque
S4. Lance likes barbeque or is a vegetarian
S5. Lance likes barbeque and watching college football on his big-screen TV
Now consider the following four pairs of statements. For each pair, first indicate whether or not you can say anything about their relative probability based solely on the information provided in this question. If you can say something about their relative probability, next indicate what you can say about it. For example, you might be able to say that one statement has a higher probability of being true than another, or you might be able to say that the two statements in a pair have the same probability.
a. S1, S2
b. S1, S5
c. S3, S4
d. S3, S5
I highlighted c as the discussion will be focused on c.
First let $A$ be the event that Lance likes barbecue and $B$ be the event that Lance is a vegetarian.
I didn't think much when I put $P(S4)>P(S3)$. My friend reminded my that it should be $P(S4)\geq P(S3)$.
I was about to agree with her, but then I noticed that if $P(S4)= P(S3)$ is true, it means $P(A\cup B)=P(A)\Rightarrow B\subset A\Rightarrow$ If Lance is a vegetarian, then he likes barbecue$\Rightarrow$ All vegetarians like barbecue, which would be wrong from common knowledge.
Therefore, I feel $P(S4)>P(S3)$ is still the correct answer. I'm not sure if I'm logically wrong or if I'm correct. Would like some comments.