Welcome to your interview.
The question is on your right: Transit Swipe Tracking. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
Design an in-memory tracking system for a public transit network that counts passenger swipe-ins and swipe-outs to calculate average travel times between stations.