$2\mathrm{.}16.$ A system has two key subsystems. The system is $"$up$"$ if both of its subsystems are functioning. Triple redundant systems are configured to provide high reliability. The overall system is operational as long as one of three systems is $"$up$."$ Let $A_{jk}$ correspond to the event "unit $k$ in system $j$ is functioning," for $j=1,2,3$ and $k=1,2.$