Make big profit

Tillo L asked:


During a sale, a shopkeeper reduced the advertised selling price of a dress by 20%, resulting in a profit of 4% over her cost of the dress. What percentage of profit would the shopkeeper have made if she had sold the dress at the original selling price?

paladin asked:


How do you use you profit figures to tell you what to do next ?

I can see that a flower seller might look at increasing average profit curve and conclude it would be profitable to buy and resell more flowers. As the curve levels out, the flower seller would know when to stop buying increasing numbers of flowers.

Same system would work for several variables but I can’t get my head around it.