diff --git a/chapters/chap15.ipynb b/chapters/chap15.ipynb
index 52fb9c8..b5f9f5f 100644
--- a/chapters/chap15.ipynb
+++ b/chapters/chap15.ipynb
@@ -409,7 +409,7 @@
     "\n",
     "    def add_time(self, hours, minutes, seconds):\n",
     "        duration = make_time(hours, minutes, seconds)\n",
-    "        seconds = time_to_int(self) + time_to_int(duration)\n",
+    "        seconds = Time.time_to_int(self) + Time.time_to_int(duration)\n",
     "        return Time.int_to_time(seconds)"
    ]
   },