HI I'm New to Developing Can you please let me Know how to write Applying Discount on Record using trigger and apex Class. please send me code i'm able to understand or below is my apex class code, please Modify it.
Public Class PenClassdemonstration {

Public Static Void applyDiscountPen (list<pen__c> varpenslistNew) {

for (pen__c varp : varpenslistNew) {

if(varp.penamount__C >= 100) {

varp.penamount__C = varp.penamount__C - 20;

   }
  }
 }
}