A retailer getting a discount of $20\%$ on the marked price sells an article at the marked price. The percentage of profit of the retailer is:

Let the Marked Price ($\text{MP}$) of the article be $100$.
The retailer buys it at a $20\%$ discount, so the Cost Price ($\text{CP}$) for the retailer is:

$$\text{CP}=100-20\%\text{ of }100=80$$

The retailer sells it at the marked price, so the Selling Price ($\text{SP}$) is:

$$\text{SP}=100$$

Profit earned by the retailer:

$$\text{Profit}=\text{SP}-\text{CP}=100-80=20$$

Profit percentage:

$$\text{Profit Percentage}=\left(\frac{\text{Profit}}{\text{CP}}\right)\times 100=\left(\frac{20}{80}\right)\times 100=25\%$$