Req of this series/parallel circuit?

[letsgo]

Homework Statement

What is the Req of this circuit?
The answer should be 150 ohms, but I can't get that.

Homework Equations

Req = R1 + R2
1/Req = 1/R1 + 1/R2

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?

 

[CWatters]

A hint..

There appears to be a wire (grey) connected from one end of R3 to the other end. What does that mean for the voltage across R3 or R3 in general?

 

[letsgo]

A hint..

There appears to be a wire (grey) connected from one end of R3 to the other end. What does that mean for the voltage across R3 or R3 in general?

No clue. It's the same as the source's voltage?

Also, did you manage to get Req?

 

[CWatters]

The grey wire has a very low resistance, let's assume it's zero Ohms. You therefore have R3 in parallel with a zero Ohm resistor..

For that bit of the circuit..

I/R = 1/R3 + 1/zero

but 1/zero = ∞

so R3 in parallel with zero Ohms = zero Ohms.

The circuit reduces to..

 

[CWatters]

I'll let you have another go at calculating Req.

 

[letsgo]

The grey wire has a very low resistance, let's assume it's zero Ohms. You therefore have R3 in parallel with a zero Ohm resistor..

For that bit of the circuit..

I/R = 1/R3 + 1/zero

but 1/zero = ∞

so R3 in parallel with zero Ohms = zero Ohms.

The circuit reduces to..

Smart, thanks.