input: Write a python function to perform basic calculator operations including addition, subtraction, multiplication, division, and modulus. 
  assert add(10, 20) == 30
  assert subtract(20, 10) == 10
  assert multiply(20, 10) == 200
  assert divide(20, 10) == 2.0
  assert modulus(20, 10) == 0
output: def add(a, b):
      return a + b
  
  def subtract(a, b):
      return a - b
  
  def multiply(a, b):
      return a * b
  
  def divide(a, b):
      return a / b
  
  def modulus(a, b):
      return a % b