Revert "Remove the undefined variable challenge."

This reverts commit 329ac65.

Author: jdblischak

analysis/challenge.Rmd

@@ -56,6 +56,26 @@ The function `read.csv()` is unable to open the data file. What's wrong with the
 path to the file? Apply what you know about absolute and relative paths to
 update the path and re-run the analysis.
 
+## Undefined variable
+
+The next error you encounter is:
+
+```
+Quitting from lines 28-31 (spotify.Rmd) 
+Error in sample.int(length(x), size, replace, prob) : 
+  object 'numTrainingSamples' not found
+Calls: <Anonymous> ... withVisible -> eval -> eval -> sample -> sample.int
+Execution halted
+```
+
+It looks like the variable `numTrainingSamples` isn't defined in the Rmd file.
+This error often occurs when a variable is interactively created in the R
+console, but you forget to define it in the script.
+
+The training set should contain 3/4 of the samples. Can you define the variable
+`numTrainingSamples`? Hint: You can obtain the number of samples with
+`nrow(spotify)`.
+
 ## Missing package
 
 The next error you encounter is: