Which length has to be taken to compute magnetic resistance?

[Marvin94]

In general I read that:

$R_m =\frac{l}{\mu A}$

But considering for example the following transformer:

I don't understand why the lengths taken from the sketch by the solution's procedure are the following:

For the right and left part resistance:
$R_r = R_l = \frac{(\frac{1}{2} + \frac{1}{2} + 1 ) l}{\mu A} = \frac{2 l}{\mu A}$

For the middle part resistance:
$R_m = \frac{l}{\mu A}$

So my question in general is: how should I measure the "l" in the general formula?

 

[Hesch]

how should I measure the "l" in the general formula?

Place two nodes at the top/bottom of the middle part. Call them A and B.
The length of the path from A to B through the left and right part = 2 * l
Through the middle part the length = l

 

[Marvin94]

Very clear. Thanks!