An electrical system has open-closed switches `S_1, S_2a n dS_3` as shown in fig. The switches operate independently of one another and the current will flow from `AtoB` either if `S_1` is closed or if both `S_2a n dS_3` are closed. If `P(S_1)=P(S_2)=P(S_3)=1//2,` then find the probability that the circuit will work. fig