What is the sum of subsystems MTBFs?

The sum of subsystems' Mean Time Between Failures (MTBFs) is not simply the arithmetic addition of their individual MTBFs, as it depends on their arrangement in the system (series or parallel). In a series configuration, the overall MTBF is determined by the weakest link, often leading to a lower overall MTBF. In a parallel configuration, the system can continue to function despite some failures, resulting in a higher overall MTBF. Thus, the calculation requires a careful consideration of the system's architecture and the reliability of each subsystem.