The system requirement would be
that equipment A and either equipment C_{1} or C_{2} work, or
that equipments B_{1} and C_{1} work, or that B_{2}
and C_{2} work for success. Equipments with the same letter are
identical, i.e., C_{1} = C_{2} and B_{1} =
B_{2}.

P_{S} = P(mission success with A working)
P_{A}
+ P(mission success with A failed) (1  P_{A}) P_{S}
= (2P_{C}  P_{C}^{2}) P_{A} +
[2P_{B} P_{C}  (P_{B} P_{C}
)^{2} ] (1  P_{A}) 
(6.30)

An example involving the above diagram is as follows:
Given that,
P_{A} = 0.3
P_{B1} =
P_{B2} = 0.1
P_{C1} =
P_{C2} = 0.2
Evaluating the probability of success for a given mission using
equation 6.30 is:
P_{S} = (.4  .04) .3 + [.04  .0004] (.7)
P_{S} =
0.13572
The equivalent series reliability mathematical model for this
system is:

P_{S} = P_{A}
P_{B}^{2} P_{C}^{2} 
(6.31) 
and the reliability is
0.00012.