Hi Anonymous!
I agree to the existing answers so far. Don't assume something especially without making it absolutely explicit in your communication to the interviewer (this is true for your specific question, but also in general for case interviews - whenever in doubt, be more explicit than you would do regularly).
From a tactical perspective during the case interview, especially giving the limited time available, I would firstly go for the easy-to-calculate/easily available numbers, and clarify explicitly with the interviewer if he prefers to have the number presented in a different way (e.g. on an annual basis) as well.
Hope that helps - if so, please give it a thumbs-up with the upvote button!
Robert