This is a very open-ende question. I believe you need to triangulate this using various factors. You can ask the interviewer about the following details of each store and then try to come up with an estimate:
- Any historical sales split information
- The size of the store in terms of how many products it sells
- The location of store in terms of whether its present in a top tier city or a lower one
- The population of cities in which these stores are located
- Presence of competitors, i.e. other such stores
You can make assumptions around mindset of people as well in a city. The bottomline is that the interviewer wants to check your thought process. So, you should consider as many factors as possible. Make some logical assumptions. They might not true but you should have a plausible explanation behind each and how they impact the sales.
Hope this helps.