diff --git a/lectures/opt_transport.md b/lectures/opt_transport.md
index e6d1ab348..a97955718 100644
--- a/lectures/opt_transport.md
+++ b/lectures/opt_transport.md
@@ -681,10 +681,12 @@ X
 Sure enough, we have the same solution and the same cost
 
 ```{code-cell} ipython3
-total_cost = np.sum(X * C)
+total_cost = np.vdot(X, C)
 total_cost
 ```
 
+Here we use [np.vdot](https://numpy.org/doc/stable/reference/generated/numpy.vdot.html) for the trace inner product of X and C
+
 ### A Larger Application
 
 Now let's try using the same package on a slightly larger application.