Ideal Batteries' Internal Resistance

[LowerMammal]

Homework Statement

A circuit is constructed with five resistors and one real battery as shown above right. We model. The real battery as an ideal emf V = 12 V in series with an internal resistance r as shown above left. The values for the resistors are: R1 = R3 = 59 Ω, R4 = R5 = 79 Ω and R2 = 140 Ω. The measured voltage across the terminals of the batery is Vbattery = 11.61 V.
What is r, the internal resistance of the battery?

Homework Equations

Vb = Vo((R/r)/(1+R/r))
Vb= voltage across battery terminals
Vo= Internal voltage
r= internal resistance
R= external resistance
Total ext. resistance in parallel: 1/Rt= 1/r1 + 1/r2
total ext. resistance in series: Rt = r1 + r2

The Attempt at a Solution

Taking total resistance dividing by the amperage.

 

[SteamKing]

No figure attached.

 

[CWatters]

Taking total resistance dividing by the amperage.

That would just give you the ideal voltage which is already known.

Hint: Potential divider