diff --git a/ipynb/Cheryl.ipynb b/ipynb/Cheryl.ipynb
index 0d7e0c6..75a68de 100644
--- a/ipynb/Cheryl.ipynb
+++ b/ipynb/Cheryl.ipynb
@@ -4,7 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "<div align=\"right\" style=\"text-align: right\"><i>Peter Norvig<br>April 2015<br>Python 3: Feb 2019<br>Steve's bus: Apr 2020</i></div>\n",
+    "<div align=\"right\" style=\"text-align: right\"><i>Peter Norvig<br>April 2015<br>Python 3: Feb 2019<br>Steve's bus: Apr 2020<br>Mad Cheryl: May 2020</i></div>\n",
     "\n",
     "# When is Cheryl's Birthday?\n",
     "\n",
@@ -482,7 +482,7 @@
    ],
    "source": [
     "times = '06:32 06:43 06:50 07:17 07:46 08:19 08:32 09:17 09:19 09:50'.split()\n",
-    "dates = [t.replace(':', ' ') for t in times]\n",
+    "dates = [time.replace(':', ' ') for time in times]\n",
     "\n",
     "cheryls_birthday()"
    ]
@@ -493,6 +493,47 @@
    "source": [
     "Steve took the 8:32 bus."
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Another New Puzzle: Evil Mad Scientist Cheryl\n",
+    "\n",
+    "![](https://norvig.com/images/cheryl-trolley.png)\n",
+    "\n",
+    "Again, we can solve this problem just by changing the format of `dates`:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 18,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "{'C 3'}"
+      ]
+     },
+     "execution_count": 18,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "pads = 'A2 A3 A6 B4 B5 C1 C3 D1 D2 D4'.split()\n",
+    "dates = [L+' '+N for L,N in pads]\n",
+    "\n",
+    "cheryls_birthday()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Mad scientist Cheryl was refering to pad C3. (But may I point out that this Cheryl is not actually a mad scientist, just a [mad engineer](https://www.evilmadscientist.com/2015/evil-mad-engineers/). A true mad scientist would kill 25 people and use the other 25 as a control group.)"
+   ]
   }
  ],
  "metadata": {