Questions like this aren't really about the specific problem or coming up with working code off the top of your head. They're looking for insight into how you think and how you approach a problem. They're not asking you to code Fibonacci because it's relevant to their actual work tasks. It's a generic challenge that any software engineer should be able to provide SOME solution to. If you're applying for a senior position like this, it's probably assumed that you have experience with a variety of languages and periodically learn new ones. Having told them that you don't currently know python, they certainly don't expect you to write good code in it. Just give it your best shot based on whatever language syntax you do know. Look up information online to fill in details or, if you can't do that in the interview, tell them that that's what you would do in a real situation. What nobody wants to hear is "This is hard." or "This isn't fair." or "I don't know what to do." At a senior level, you're expected to solve problems, not just complain about them.