title: I want to write a c# logic for a discount related purchase option
body: <p>Here is the scenario.</p>
  
  <p>There is a product (eg. Pen). Each pen costs 10$.For every 4 pens i buy, i get 10$ off. So if i buy 5 pens, I should be charged 40$. If purchased quantity is less than 4 there is no freebie. If someone buys more than 4 like 8, so the total charge should be 60$ and so on.</p>
  
  <p>I require the logic in C#.</p>
  
  <p>Thanks.</p>