title: .groupby & .fillna with median
body: <pre><code># Create a groupby object: by_sex_class
  by_sex_class = titanic.groupby(["sex","pclass"]).count()
  
  # Write a function that imputes median
  def impute_median(series):
      return series.fillna(series.median())
  
  # Impute age and assign to titanic['age']
  titanic.age = by_sex_class["age"].transform(impute_median)
  
  # Print the output of titanic.tail(10)
  print(titanic.tail(10))
  </code></pre>
  
  <p>I'm unclear how can we assign the column, ["age"] from modified (grouped) df, by_sex_class, to the original (un-grouped) df, titanic. </p>
  
  <p>Wouldn't the assignments be jumbled up?</p>
  
  <p>Thanks in advance for your explanations.</p>