Theory:

What is it mean by discount:

To increase the sale and also to clear the old stock during the festival seasons, shopkeepers offer a certain percentage of rebates on the marked price of the articles. In other words, this rebate is known as a discount.
The reduction in market price to increase the sale or to dispose of old goods is known as the discount.
Usually, discounts are express as a percentage of the marked price. The customer or buyer pays the difference between the Marked price and the discounted price.

Thus, we have $\mathit{Discount}=\mathit{Marked}\phantom{\rule{0.147em}{0ex}}\mathit{Price}×\mathit{Rate}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{discount}$

What is marked price and selling price?
• Actually, In big shops and departmental stores, we see that every product is tagged with a card with a price written on it. The price marked on it is called the marked price.
• Based on this marked price, the shopkeeper offers a discount of a certain percentage. The price payable by the customer after deduction of discount is called the Selling price(S.P).
Therefore $S.P=\mathit{Marked}\phantom{\rule{0.147em}{0ex}}\mathit{price}-\mathit{Discount}$