Welcome to your interview.
The question is on your right: Scalable Transaction Messaging and Pricing. 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.
How would you design a modular system that prints transaction messages for a vending machine, and how would you scale this system to optimize for cheapest drink prices across a complex rewards scheme?