input: Write a python function to perform addition and subtraction using the `Numbers` class.
  assert perform_operations(10, 5) == (15, 5)
  assert perform_operations(20, 10) == (30, 10)
  assert perform_operations(7, 3) == (10, 4)
output: class Numbers:
      def __init__(self, a, b):
          self.a = a
          self.b = b
  
      def add(self):
          return self.a + self.b
  
      def subtract(self):
          return self.a - self.b
  
  def perform_operations(a, b):
      numbers = Numbers(a, b)
      return (numbers.add(), numbers.subtract())