Step 1: Understanding the Concept:
When voltage and current are in phase (\(\phi = 0\)): circuit is purely resistive, no reactance present. Impedance: \(Z = \frac{E_{\rm rms}}{I_{\rm rms}}\).
Step 2: Detailed Explanation:
Given peak values: \(Z = \frac{E_0}{I_0} = \frac{100}{5} = 20\,\Omega\). Since \(\phi = 0\): \(R = Z = 20\,\Omega\). Reactance: \(X = Z\sin\phi = 0\). Thus, the circuit is purely resistive and impedance equals resistance.
Step 3: Final Answer:
\[
\boxed{Z = 20\,\Omega,\quad R = 20\,\Omega,\quad X = 0\,\Omega}
\]