Consider the system of components connected as in the accompanying picture. components 1 and 2 are connected in parallel, so that subsystem works iff either 1 or 2 works; since 3 and 4 are connected in series, that subsystem works iff both 3 and 4 work. if components work independently of one another and p(component i works) 5 .9 for i 5 1,2 and 5 .8 for i 5 3,4, calculate p(system works).