Tryed to solve 21 in ipython.

ipython/EulerProblem020.ipynb

@@ -0,0 +1,87 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Euler Problem 20\n",
+    "\n",
+    "n! means n × (n − 1) × ... × 3 × 2 × 1\n",
+    "\n",
+    "For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,\n",
+    "and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.\n",
+    "\n",
+    "Find the sum of the digits in the number 100!"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Write our own factorial implementation and get the solution."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def factorial(n):\n",
+    "    assert(n > 0)\n",
+    "    if n == 1:\n",
+    "        return 1\n",
+    "    else:\n",
+    "        return n * factorial(n - 1)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "648\n"
+     ]
+    }
+   ],
+   "source": [
+    "f100 = factorial(100)\n",
+    "print(sum(map(int, str(f100))))\n",
+    "assert(sum(map(int, str(f100))) == 648)"
+   ]
+  }
+ ],
+ "metadata": {
+  "completion_date": "Fri, 5 Sep 2014, 10:42",
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.5.4"
+  },
+  "tags": [
+   "factorial"
+  ]
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}