Parallel versus serial

The availability numbers that we discussed are for individual components in a system. For example, you may do extensive testing and analysis of your software, and find that a particular component has a certain availability number. But that's not the whole story — your component is part of a larger system, and will affect the availability of the system as a whole. Consider a system that has several modules. If module A relies on the services of module B, and both modules have a five nines availability (99.999%), what happens when you combine them? What's the availability of the system?