Define success for the Extra Member feature
Measure successMediumProblem. Netflix shipped Extra Member slots (a paying account can add someone outside the household for a fee). A PM asks you to define what success looks like and how you would measure it.
Before you reveal: say your answer out loud, as if you were in the real interview — get your reasoning across clearly first. There is no single correct answer: reading what the interviewer is really after and defending your own thinking is what makes an answer strong.