text: class Student():
      def __init__(self,a,b):
          self.a=a
          self.b=b
      
      def sum(self,a=None,b=None,c=None):
          s=0
          if(a!=None and b!=None and c!=None):
              s=a+b+c
          elif (a!=None and b!=None):
               s=a+b
          else:
               s=c
          return s
  
  s1=Student(50,30)
  print(s1.sum(20,30))
  print(s1.sum(20,30,10))
  print(s1.sum(20))
  