Consider the series-parallel networks depicted below. Each system comprises four components. The time-to-failure for components (1) and (2) is exponentially distributed with a mean of one year; the time-to-failure for components (3) and (4) is exponentially distributed with a mean of two years. All failures are assumed to be statistically independent.
Note that Network A fails if (component 1 or3 fails) and(component 2 or4 fails); Network B fails if (component 1and2 fail) or (component 3 and4 fails).
(i) Estimate the mean of the time-to-failure for Network A, the time-to-failure for Network B.