class Student:

  def __init__(self, name, marks):
    self.name = name
    self.marks = marks
    self.grade = ""

  def calculateGrade(self):
    if self.marks <40: 
      self.grade = "D"
    elif self.marks < 60:
      self.grade = "C"
    elif self.marks < 80:
      self.grade = "B"
    else:
      self.grade = "A"

  def displayGrade(self):
    print(self.grade)


st1 = Student("St1", 10)
st2 = Student("St2", 75)

st1.calculateGrade()
st1.displayGrade()

st2.calculateGrade()
st2.displayGrade()