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NAs
Factors
Summary
27. Exercise 2

## Instruction

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).

## Exercise

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

### Stuck? Here's a hint!

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

sum(is.na(houses$last_renovation) & is.na(houses$built))