Step 1: Compute distributable profit.
Total profit $=₹ 1{,}80{,}000$, reserve $=₹ 30{,}000$
\Rightarrow\ Distributable profit $=₹(1{,}80{,}000-30{,}000)=₹ 1{,}50{,}000$.
Step 2: Pay preferred dividend first.
Preferred dividend rate $=20\%$ on par ₹ 10 $\Rightarrow$ ₹ 2 per preferred share.
Total preferred dividend $=50{,}000\times ₹ 2=₹ 1{,}00{,}000$.
Step 3: Amount left for common shareholders.
Left for common $=₹(1{,}50{,}000-1{,}00{,}000)=₹ 50{,}000$.
Step 4: Dividend per common share and percent.
Common shares $=20{,}000$ \Rightarrow\ dividend per common share $=\dfrac{₹ 50{,}000}{20{,}000}=₹ 2.50$.
Percent on par ₹ 10 $=\dfrac{2.50}{10}\times 100=25\%$.
\[
\boxed{25\%}
\]