title: How to sum the values of a nested dictionary
body: <p>I am working on some homework and I need define a function addDict(d) which adds up the values in a dictionary that looks like the following:</p>
  
  <pre><code>d = {'Mon':{'355':2,'451':1,'360':2},'Tue':{'451':2,'360':3},
  'Thu':{'355':3,'451':2,'360':3}, 'Fri':{'355':2}, Sun':{'355':1,'451':3,'360':1}}
  </code></pre>
  
  <p>I'm having a tough time trying to figure out how I can access the values in the nested dictionaries so that I can add them up and return a single dictionary mapping those sums to the classes such as {'355': 8, '451': 8, '360': 9}</p>