title: How to delete columns with at least 20% missing values
body: <p>Is there an efficient way to delete columns that have at least 20% missing values?</p>
  
  <p>Suppose my dataframe is like:</p>
  
  <pre><code>   A      B      C      D
  0  sg     hh     1      7
  1  gf                   9
  2  hh                   10
  3  dd                   8
  4                       6 
  5  y                    8`
  </code></pre>
  
  <p>After removing the columns, the dataframe becomes like this:</p>
  
  <pre><code>   A       D
  0  sg      7
  1  gf      9
  2  hh      10
  3  dd      8
  4          6 
  5  y       8`
  </code></pre>