Add comment about reducing duplicate code

foundations/02-class-id-selectors/solution/solution.css

@@ -12,6 +12,15 @@
   font-size: 36px;
 }
 
+/*
+    In the id selector 'four' below, we could have also
+    added a rule to set the font size to 24px.
+
+    However, since the elements 'Number 3' and 'Number 4'
+    in the HTML file share the same font size, we reused
+    the 'adjust-font-size' class above to help reduce
+    duplicate code.
+*/
 #four {
   background-color: hsl(120, 100%, 75%);
   font-weight: bold;