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diff --git a/backup/Modern_Physics_version_backup/chapter12.ipynb b/backup/Modern_Physics_version_backup/chapter12.ipynb
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--- a/backup/Modern_Physics_version_backup/chapter12.ipynb
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@@ -1,63 +0,0 @@
-{
- "metadata": {
-  "name": "",
-  "signature": "sha256:3ee27523440d66319c8d17b88b593002a8ca441a824b734c02da7b3a1165c0fa"
- },
- "nbformat": 3,
- "nbformat_minor": 0,
- "worksheets": [
-  {
-   "cells": [
-    {
-     "cell_type": "heading",
-     "level": 1,
-     "metadata": {},
-     "source": [
-      "12: Artificial Radioactivity"
-     ]
-    },
-    {
-     "cell_type": "heading",
-     "level": 1,
-     "metadata": {},
-     "source": [
-      "Example number 12.1, Page number 247"
-     ]
-    },
-    {
-     "cell_type": "code",
-     "collapsed": false,
-     "input": [
-      "#import modules\n",
-      "import math\n",
-      "from __future__ import division\n",
-      "\n",
-      "#Variable declaration\n",
-      "r=0.5;      #ratio of mass of Pb206 and mass of U238\n",
-      "t=4.5*10**9;     #half life(years)\n",
-      "\n",
-      "#Calculation\n",
-      "T=(math.log(1+r))*(t/0.693);        #age(years)\n",
-      "T=T/10**9;\n",
-      "\n",
-      "#Result\n",
-      "print \"age of rock is\",round(T,2),\"*10**9 years\""
-     ],
-     "language": "python",
-     "metadata": {},
-     "outputs": [
-      {
-       "output_type": "stream",
-       "stream": "stdout",
-       "text": [
-        "age of rock is 2.63 *10**9 years\n"
-       ]
-      }
-     ],
-     "prompt_number": 5
-    }
-   ],
-   "metadata": {}
-  }
- ]
-}
-\ No newline at end of file