Cascade Layers: New task

Author: estelle

learn/tasks/cascade/marking.md

@@ -22,3 +22,6 @@ One possible solution is as follows:
  @layer yellow, green, purple;
 ```
 
+There is one thing the student needs to do in this task: change the order of precedence so the declaration for the desired color is in the last declared layer, which is what his solution shows.
+
+The student needs to remember that unlayered normal styles take precedence over layered normal styles. But, if all styles are within layers, as in the case of this task, styles in later declared layers take precedence over styles declared in earlier layers. Moving the purple layer to end means it has precedence over the green and yellow layers.