title: Python TypeError: %d format: a number is required, not NoneType
body: <p>I am newbee trying to learn python using LPTH. I trying to do exercise 21 in a different way. Instead of putting the values in the variable, I am asking the user to put the value.</p>
  
  <pre><code>def add(a,b):
      print "Add %d and %d." %(a,b)
      print a+b
  
  def substract (a,b):
      print "Substract %d from %d." %(a,b)
      print a-b
  
  def multiply (a,b):
      print "Multiply %d*%d." %(a,b)
      print a*b
  
  def divide (a,b):
      print "Dividing %d and %d." %(a,b)
      print a/b
  
  total_age_of_the_class = add(int(raw_input("&gt;")) , int(raw_input("&gt;")))
  #height_difference = substract (int(raw_input("Ram's height:"), int(raw_input("Shyam's height:"))))
  height = substract (78,4)
  weight = multiply (90,2)
  iq = divide (100,2)
  
  print "Age:%d, height:%d, weight:%d, iq:%d" %(total_age_of_the_class, height, weight, iq)
  </code></pre>
  
  <p>For some reason it throws an error when the code run the last line of the code.
  Thanks in advance for your help.</p>