diff --git a/source/calculus/source/06-AI/07.ptx b/source/calculus/source/06-AI/07.ptx
index a8f3e8573..9c33f39e6 100644
--- a/source/calculus/source/06-AI/07.ptx
+++ b/source/calculus/source/06-AI/07.ptx
@@ -165,13 +165,13 @@
             <task>
                 <statement>
                     <p>
-                        Draw and label a horizontal rectangle across the middle of the plate of width <m> l_i</m>
-                        and height <m> x_i</m>. What is the area <m>A_i</m> of this rectangle?
+                        Draw and label a thin, horizontal rectangle across the middle of the plate of width <m> l_i</m>
+                        and height <m> \Delta x_i</m>. What is the area <m>A_i</m> of this rectangle?
                     </p>
                 </statement>
                 <answer>
                     <p>
-                        <m>A_i=l_ix_i</m>
+                        <m>A_i=l_i\Delta x_i</m>
                     </p>
                 </answer>
             </task>