added consistency to 04-data-types-and-format

Added Challenge guidelines (taken from the lesson) and language markup to section 04-data-types-and-format for consistency with the other sections.

Author: NPalopoli

_extras/guide.md

@@ -245,13 +245,20 @@ stack_selection.columns = stack_selection.columns.droplevel()
 
 ### Challenge - Changing Types
 
+* Try converting the column `plot_id` to floats using `surveys_df.plot_id.astype("float")`. Next try converting `weight` to an integer. What goes wrong here? What is Pandas telling you?
+
 Pandas cannot convert types from float to int if the column contains NaN values.
 
 ### Challenge - Counting
+
+* Count the number of missing values per column. Hint: The method .count() gives you the number of non-NA observations per column. Try looking to the .isnull() method.
+
+~~~
 surveys_df.isnull()
+~~~
+{: .language-python}
 
-If the students have trouble generating the output, or anything happens with that, there is a file
-called "sample output" that contains the data file they should generate.
+If the students have trouble generating the output, or anything happens with that, there is a file called "sample output" that contains the data file they should generate.
 
 ## 05-merging-data