Early in my career, when I first started interviewing, I used to ask a version of this to recent grads. It was never a make-or-break question, but I found it to be a great way to a.) see how people approach problem solving and probability and b.) see how they respond when you start asking whys (even if they answered/guessed 1/3). It's something that takes zero code to answer, and the intuition is easy to grok once explained.

The other part I particularly enjoyed was the people who initially guessed wrong, but then got to the answer intuitively almost always sent me code proving the answer.

For the record, my question was: "Two points are randomly and uniformly selected on a line 0.0 to 1.0. What is the most probable distance between the two points?"