You want a system to have at least 99.5% reliability.  The s…

You want a system to have at least 99.5% reliability.  The system is to be implemented using replicated instances of a component, but a single instance of that component has only 90% reliability.  If the system will work as long as at least one of the components is working, no other failures are relevant, and component failures are independent, what is the minimum number of component instances (total, including the first component) that must be in the system?