Real, complex systems will have a variety of parallel and serial flows within them. The way to calculate the availability of the entire system is to work with subcomponents. Take the availability numbers of each component, and draw a large diagram with these numbers, making sure that your diagram indicates parallel and serial flows. Then take the serial flows, and collapse them into an aggregate sub-system using the formula. Do the same for parallel flows. Keep doing this until your system has just one flow — that flow will now have the availability number of the entire system.