The adjoining figure shows an AC circuit with resistance $R$, inductance $L$ and source voltage $V_S$, Then, 

As the phasors $V_R$ and $V_L$ are perpendicular to each other, their resultant

$\begin{array}{rcl}{V}_S& =& \sqrt{V_R^{ 2}+V_L^{ 2}}\\ & =& \sqrt{{\left(70\right)}^2+{\left(20\right)}^2}\\ & =& 72.8 V\end{array}$

If $\phi$ is the phase angle between current and source voltage,

$$\begin{gathered} \tan \phi =\frac{X_L}{R}=\frac{V_L}{V_R}=\frac{20}{70}=\frac{2}{7} \\ \Rightarrow \phi ={\tan }^{-1}\left(\frac{2}{7}\right) \end{gathered}$$