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27. Exercise 2


As you saw, the last_renovation column contains some missing values. A missing value under this column indicates that a property was never renovated. We will replace NAs with the year in which a property was build (the built column).


Before we replace the NAs in the last_renovation column, let's check the number of houses which have missing values both in built and last_renovation columns.

Stuck? Here's a hint!

You'll have to to use the function inside the sum() function for both columns:

sum($last_renovation) &$built))