1. The problem statement, all variables and given/known data What is the Req of this circuit? The answer should be 150 ohms, but I can't get that. 2. Relevant equations Req = R1 + R2 1/Req = 1/R1 + 1/R2 3. The attempt at a solution If I treat R1 and R3 as series, I get R13= 200 ohms. Then if I treat R13 and R2 and parallel I get R123 = 66.7 ohms. Then if I treat R123 and R4 as series, I get Req = 266.7 ohms. Also, since the current is 0 (open switch), then the potential difference between points P and Q would also be 0, right?