title: I don't understand this error when I try to use operators on %s on Python 2.7
body: <p>I started out programming a few days ago. I did some course on the internet and I was now looking to make a simple program to solve a rule of three (cross multiplication) 
  it goes as follow : </p>
  
  <pre><code>    # Def variables
  
      print "a/b = c/d"
      print "Put in the values you know. Leave the one that you dont know with a '?' "
      a = raw_input ("A = ")
      b = raw_input ("B = ")
      c = raw_input ("C = ")
      d = raw_input ("D = ")
  
      # if statements
  
      if a == "?":
          "%s * %s / %s" % (c, b, d) = "a"
          print "a"
  
       # in work elif b == "?" 
  </code></pre>
  
  <p>when I run it, it says "*** can't assign to operator, line 13"
  it seems to be a syntax error, but I really don't get it. Thanks for your patience :) Sorry for my bad english</p>