Answer
5.2 $\Omega$
Work Step by Step
N=12, $\Delta \Phi=(4.0-9.0)Wb= -5.0 Wb$,
$\Delta t= 0.050s$ and I=230 A.
$\epsilon=-N\frac{\Delta \Phi}{\Delta t}= -12\times\frac{-5.0 Wb}{0.050 s}= 1200 V$
Thus, we find:
Resistance r=$\frac{\epsilon}{I}=\frac{1200 V}{230 A}= 5.2 \Omega$
