title: Create a column that divides the other 2 columns using Pandas Apply()
body: <p>Given a dataset -</p>
  <pre><code>country     year    cases   population
  Afghanistan 1999    745     19987071
  Brazil      1999    37737   172006362
  China       1999    212258  1272915272
  Afghanistan 2000    2666    20595360
  Brazil      2000    80488   174504898
  China       2000    213766  1280428583
  </code></pre>
  <p>The task is to get the ratio of cases to population using the pandas apply function, in a new column called &quot;prevalence&quot;</p>
  <p>This is what I have written</p>
  <pre><code>def calc_prevalence(G):
      assert 'cases' in G.columns and 'population' in G.columns
  
      G_copy = G.copy()
      G_copy['prevalence'] = G_copy['cases','population'].apply(lambda x: (x['cases']/x['population']))
      display(G_copy)
  </code></pre>
  <p>but I am getting a</p>
  <pre><code>KeyError: ('cases', 'population')
  </code></pre>