title: For loop to drop columns based on null values
body: <p>Hello I have a data frame called lc. And in the shape of the data frame is (235607,146)
  I was able to write a code that shows me the percentage of null values in each column(<code>np.sum(lc.isnull())/lc.shape[0]*100</code>). And now I need help in writing a for loop that drops all the columns with null values greater than 70 percent </p>