diff --git a/machine_learning/3_classification/lecture/week4/.ipynb_checkpoints/quiz-Preventing Overfitting in Decision Trees-checkpoint.ipynb b/machine_learning/3_classification/lecture/week4/.ipynb_checkpoints/quiz-Preventing Overfitting in Decision Trees-checkpoint.ipynb
index 98d5aa4..249904c 100644
--- a/machine_learning/3_classification/lecture/week4/.ipynb_checkpoints/quiz-Preventing Overfitting in Decision Trees-checkpoint.ipynb	
+++ b/machine_learning/3_classification/lecture/week4/.ipynb_checkpoints/quiz-Preventing Overfitting in Decision Trees-checkpoint.ipynb	
@@ -127,7 +127,6 @@
    "metadata": {},
    "source": [
     "<img src=\"images/lec4_quiz01_pic07.png\">\n",
-    "<img src=\"images/lec4_quiz01_pic07-01.png\">\n",
     "\n",
     "*Screenshot taken from [Coursera](https://www.coursera.org/learn/ml-classification/exam/NDTdJ/preventing-overfitting-in-decision-trees)*\n",
     "\n",
@@ -146,13 +145,25 @@
    "metadata": {},
    "source": [
     "<img src=\"images/lec4_quiz01_pic08.png\">\n",
-    "<img src=\"images/lec4_quiz01_pic08-01.png\">\n",
     "\n",
     "*Screenshot taken from [Coursera](https://www.coursera.org/learn/ml-classification/exam/NDTdJ/preventing-overfitting-in-decision-trees)*\n",
     "\n",
     "<!--TEASER_END-->"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "** Answer **\n",
+    "\n",
+    "At this node, the y values have:\n",
+    "- 3 True (+1)\n",
+    "- 1 False (-1)\n",
+    "\n",
+    "The classification error is: 1/4 = 0.25"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -171,6 +182,50 @@
     "<!--TEASER_END-->"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "** Answer **\n",
+    "- x1 has 2 zeros and 2 ones, so spliting by x1, we have the tree structure as follow"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 47,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "                        root (3, 1)\n",
+      "         |---------------|----------------|\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "      node_1 (2,0)                  node_1 (1,1) \n"
+     ]
+    }
+   ],
+   "source": [
+    "print '                        %s' % 'root (3, 1)'\n",
+    "print '         |---------------|----------------|'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '      node_1 (2,0)                  node_1 (1,1) '"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Classification error = 1/4 = 0.25**"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -183,13 +238,56 @@
    "metadata": {},
    "source": [
     "<img src=\"images/lec4_quiz01_pic10.png\">\n",
-    "<img src=\"images/lec4_quiz01_pic10-01.png\">\n",
     "\n",
     "*Screenshot taken from [Coursera](https://www.coursera.org/learn/ml-classification/exam/NDTdJ/preventing-overfitting-in-decision-trees)*\n",
     "\n",
     "<!--TEASER_END-->"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "** Answer **\n",
+    "- x2 has 1 zeros and 3 ones, so spliting by x2, we have the tree structure as follow"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 48,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "                        root (3, 1)\n",
+      "         |---------------|----------------|\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "      node_1 (2,1)                  node_1 (1,0) \n"
+     ]
+    }
+   ],
+   "source": [
+    "print '                        %s' % 'root (3, 1)'\n",
+    "print '         |---------------|----------------|'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '      node_1 (2,1)                  node_1 (1,0) '"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Classification error = 1/4 = 0.25**"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
diff --git a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic06-01.png b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic06-01.png
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diff --git a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic06.png b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic06.png
index 59a9991..54f1e36 100644
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diff --git a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic07-01.png b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic07-01.png
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index 6834fce..5d6f85c 100644
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diff --git a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic08-01.png b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic08-01.png
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index ef83dd1..d4a504b 100644
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diff --git a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic10-01.png b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic10-01.png
deleted file mode 100644
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diff --git a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic10.png b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic10.png
index 315aa04..288deda 100644
Binary files a/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic10.png and b/machine_learning/3_classification/lecture/week4/images/lec4_quiz01_pic10.png differ
diff --git a/machine_learning/3_classification/lecture/week4/quiz-Preventing Overfitting in Decision Trees.ipynb b/machine_learning/3_classification/lecture/week4/quiz-Preventing Overfitting in Decision Trees.ipynb
index 98d5aa4..249904c 100644
--- a/machine_learning/3_classification/lecture/week4/quiz-Preventing Overfitting in Decision Trees.ipynb	
+++ b/machine_learning/3_classification/lecture/week4/quiz-Preventing Overfitting in Decision Trees.ipynb	
@@ -127,7 +127,6 @@
    "metadata": {},
    "source": [
     "<img src=\"images/lec4_quiz01_pic07.png\">\n",
-    "<img src=\"images/lec4_quiz01_pic07-01.png\">\n",
     "\n",
     "*Screenshot taken from [Coursera](https://www.coursera.org/learn/ml-classification/exam/NDTdJ/preventing-overfitting-in-decision-trees)*\n",
     "\n",
@@ -146,13 +145,25 @@
    "metadata": {},
    "source": [
     "<img src=\"images/lec4_quiz01_pic08.png\">\n",
-    "<img src=\"images/lec4_quiz01_pic08-01.png\">\n",
     "\n",
     "*Screenshot taken from [Coursera](https://www.coursera.org/learn/ml-classification/exam/NDTdJ/preventing-overfitting-in-decision-trees)*\n",
     "\n",
     "<!--TEASER_END-->"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "** Answer **\n",
+    "\n",
+    "At this node, the y values have:\n",
+    "- 3 True (+1)\n",
+    "- 1 False (-1)\n",
+    "\n",
+    "The classification error is: 1/4 = 0.25"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -171,6 +182,50 @@
     "<!--TEASER_END-->"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "** Answer **\n",
+    "- x1 has 2 zeros and 2 ones, so spliting by x1, we have the tree structure as follow"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 47,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "                        root (3, 1)\n",
+      "         |---------------|----------------|\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "      node_1 (2,0)                  node_1 (1,1) \n"
+     ]
+    }
+   ],
+   "source": [
+    "print '                        %s' % 'root (3, 1)'\n",
+    "print '         |---------------|----------------|'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '      node_1 (2,0)                  node_1 (1,1) '"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Classification error = 1/4 = 0.25**"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -183,13 +238,56 @@
    "metadata": {},
    "source": [
     "<img src=\"images/lec4_quiz01_pic10.png\">\n",
-    "<img src=\"images/lec4_quiz01_pic10-01.png\">\n",
     "\n",
     "*Screenshot taken from [Coursera](https://www.coursera.org/learn/ml-classification/exam/NDTdJ/preventing-overfitting-in-decision-trees)*\n",
     "\n",
     "<!--TEASER_END-->"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "** Answer **\n",
+    "- x2 has 1 zeros and 3 ones, so spliting by x2, we have the tree structure as follow"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 48,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "                        root (3, 1)\n",
+      "         |---------------|----------------|\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "         |                                |\n",
+      "      node_1 (2,1)                  node_1 (1,0) \n"
+     ]
+    }
+   ],
+   "source": [
+    "print '                        %s' % 'root (3, 1)'\n",
+    "print '         |---------------|----------------|'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '         |                                |'\n",
+    "print '      node_1 (2,1)                  node_1 (1,0) '"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Classification error = 1/4 = 0.25**"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},