I once got that question in an interview for a small startup and told the interviewer: with all due respect what does that have to do with the job I’m going to do and we moved on to the next question (still passed).
If someone tells you not to do it recursively, you should be able to figure that out too.
Interview nerves might get in your way, but it’s not a trick question you need to memorize.
"Let's implement a function to return us the Nth fibonnaci number.To get a fib (fibonacci) number you add the two previous numbers, so fib(N)=fib(N-1)+fib(N+2). The starting points are fib(0)=1 and fib(1)=1. Let's assume the N is never too big (no bigger than 20)."
And that's a problem if they can't solve it.
OTOH about 15 years ago I heard from a friend that interviewed candidates that some people couldn't even count all the instances of 'a' in a string. So in fact not much has changed, except that it's harder to spot these kind of people.