Markup is the difference between a product's selling price and cost as a percentage of the cost.
If we call the cost of the dress as x, the markup is given by the following expression

The percent markup on the cost price of a dress is the same as the cost price in dollars, therefore, we can rewrite the previous expression as:

Solving for x, we have:

Since we can't have a negative cost price, the markup is 20% and the cost price of the dress is $20.