Step 1: Write the expression for phase angle in a series RLC circuit.
The phase angle is given by
\[
\tan\phi=\frac{X_L-X_C}{R}
\]
where,
\[
X_L=\omega L
\]
is inductive reactance and
\[
X_C=\frac{1}{\omega C}
\]
is capacitive reactance.
Step 2: Calculate inductive reactance.
Given,
\[
\omega=400\,\text{rad s}^{-1}
\]
\[
L=500\,\text{mH}=0.5\,\text{H}
\]
Therefore,
\[
X_L=\omega L
\]
\[
X_L=400\times0.5
\]
\[
X_L=200\,\Omega
\]
Step 3: Calculate capacitive reactance.
Given,
\[
C=20\,\mu\text{F}=20\times10^{-6}\,\text{F}
\]
Thus,
\[
X_C=\frac{1}{\omega C}
\]
\[
X_C=\frac{1}{400\times20\times10^{-6}}
\]
\[
X_C=\frac{1}{8\times10^{-3}}
\]
\[
X_C=125\,\Omega
\]
Step 4: Calculate phase angle.
Given resistance,
\[
R=150\,\Omega
\]
Using
\[
\tan\phi=\frac{X_L-X_C}{R}
\]
\[
\tan\phi=\frac{200-125}{150}
\]
\[
\tan\phi=\frac{75}{150}
\]
\[
\tan\phi=0.5
\]
Hence,
\[
\phi=\tan^{-1}(0.5)
\]
Step 5: Final conclusion.
Therefore, the phase angle between current and applied voltage is
\[
\boxed{\tan^{-1}(0.5)}
\]