diff --git a/NUCE_2100/.ipynb_checkpoints/HW3-checkpoint.ipynb b/NUCE_2100/.ipynb_checkpoints/HW3-checkpoint.ipynb
index dae1086..10ec2e3 100644
--- a/NUCE_2100/.ipynb_checkpoints/HW3-checkpoint.ipynb
+++ b/NUCE_2100/.ipynb_checkpoints/HW3-checkpoint.ipynb
@@ -118,19 +118,41 @@
    "source": [
     "---\n",
     "##  Question 2\n",
-    "A beam o f2 MeV neutrons is incident on a slab of heavy water (D2O). The total cross sections of deuterium and oxygen at this energy are 2.6 b and 1.6 b, respectively.\n",
+    "A beam of 2 MeV neutrons is incident on a slab of heavy water (D2O). The total cross sections of deuterium and oxygen at this energy are 2.6 b and 1.6 b, respectively.\n",
     "\n",
     "### Part A\n",
-    "What is the macroscopic total cross section of D2O at 2 MeV?"
+    "What is the macroscopic total cross section of D2O at 2 MeV?\n",
+    "\n",
+    "[Heavy water properties](https://srd.nist.gov/jpcrdreprint/1.555714.pdf)\n",
+    "\n",
+    "[Heavy water molar mass](https://www.acs.org/molecule-of-the-week/archive/d/deuterium-oxide.html)"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 3,
    "id": "adc08334-42a5-4b84-8d91-c28a877d1ee5",
    "metadata": {},
-   "outputs": [],
-   "source": []
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "The macroscopic total cross section of D2O is 2.270e-01 1/cm\n"
+     ]
+    }
+   ],
+   "source": [
+    "d2o_cross_section = (2*2.6+1.6)*1e-24 #cm^2\n",
+    "d2o_rho = 1.11 #g/cm^3\n",
+    "d2o_mass = 20.03 #g/mol\n",
+    "avo_no = 6.023e23 #atoms / mol\n",
+    "\n",
+    "N = avo_no * d2o_rho / d2o_mass\n",
+    "\n",
+    "d2o_sigma = N * d2o_cross_section\n",
+    "print(f\"The macroscopic total cross section of D2O is {d2o_sigma:.3e} 1/cm\")"
+   ]
   },
   {
    "cell_type": "markdown",
diff --git a/NUCE_2100/HW3.ipynb b/NUCE_2100/HW3.ipynb
index dae1086..10ec2e3 100644
--- a/NUCE_2100/HW3.ipynb
+++ b/NUCE_2100/HW3.ipynb
@@ -118,19 +118,41 @@
    "source": [
     "---\n",
     "##  Question 2\n",
-    "A beam o f2 MeV neutrons is incident on a slab of heavy water (D2O). The total cross sections of deuterium and oxygen at this energy are 2.6 b and 1.6 b, respectively.\n",
+    "A beam of 2 MeV neutrons is incident on a slab of heavy water (D2O). The total cross sections of deuterium and oxygen at this energy are 2.6 b and 1.6 b, respectively.\n",
     "\n",
     "### Part A\n",
-    "What is the macroscopic total cross section of D2O at 2 MeV?"
+    "What is the macroscopic total cross section of D2O at 2 MeV?\n",
+    "\n",
+    "[Heavy water properties](https://srd.nist.gov/jpcrdreprint/1.555714.pdf)\n",
+    "\n",
+    "[Heavy water molar mass](https://www.acs.org/molecule-of-the-week/archive/d/deuterium-oxide.html)"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 3,
    "id": "adc08334-42a5-4b84-8d91-c28a877d1ee5",
    "metadata": {},
-   "outputs": [],
-   "source": []
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "The macroscopic total cross section of D2O is 2.270e-01 1/cm\n"
+     ]
+    }
+   ],
+   "source": [
+    "d2o_cross_section = (2*2.6+1.6)*1e-24 #cm^2\n",
+    "d2o_rho = 1.11 #g/cm^3\n",
+    "d2o_mass = 20.03 #g/mol\n",
+    "avo_no = 6.023e23 #atoms / mol\n",
+    "\n",
+    "N = avo_no * d2o_rho / d2o_mass\n",
+    "\n",
+    "d2o_sigma = N * d2o_cross_section\n",
+    "print(f\"The macroscopic total cross section of D2O is {d2o_sigma:.3e} 1/cm\")"
+   ]
   },
   {
    "cell_type": "markdown",