Traders while selling goods, add a certain percentage on the cost price. This addition is called percentage mark up, and price thus obtained is called as the marked price (this is also the price printed on the product in the shop).
The operative relationship is
Cost Price + Mark Up = Marked Price
or CP + % Mark up on CP = Marked Price
The product is normally sold at the marked price in which case the marked price = the selling price.
If the shopkeeper gives a discount, he does so on the marked price and
after the discount the product is sold at its discounted price.
Hence the following relationship operates:
CP + % Mark Up (calculated on CP) = Marked Price
Marked Price - % Discount = Selling Price