text: When you have systems to analyze like this, the easiest thing is to start brainstorming on many post-its and grouping them at the end as inputs, outputs, etc. Relate to personal experience. If you think about your personally using a mass transit system, you have to think about your total experience and the point of view of the transit system. For a subway system, one usually goes into a subway where there is usually some booth there selling subway tokens or subway passes. You pay real money and get the subway tokens. You pay with these subway tokens to actually get onto the subway and then you need to look at a map to see where you are and where you need to get to. You go to the correct train and wait for it to show up. Once it does, you get on. You ride to your destination with no delays, then you get off and get out of that subway station and go your merry way. Now, that we've gone through the subway experience, you need to identify the "players" in your experience with the system. The players are you, money, subway tokens, map, train, train schedule, train on-time arrival. Inputs can be you (or people using subway) and real money, because thats where everything starts. An Output could be arriving at your correct destination on-time. Do you have the idea? I'll let you get back to your HW.