Many interviewers ask you this type of behavioral questions to see an analyst’s thought process without the help of computers and data sets. After all, technology is only as good and reliable as the people behind it. In your answer include: how you identified the variables, how you communicated them, and ideas you had to find the answer.
This example answer touches on all these points:
“First, I would gather data on how many people live in Paris, how many tourists visit in May, and their average length of stay. I’d break down the numbers by age, gender, and income, and find the numbers on how many vacation days and bank holidays there are in France. I’d also figure out if the tourist office had any data I could look at.”