diff --git a/100_Numpy_exercises.ipynb b/100_Numpy_exercises.ipynb
index 7315491..67e6a4d 100644
--- a/100_Numpy_exercises.ipynb
+++ b/100_Numpy_exercises.ipynb
@@ -339,6 +339,7 @@
     "print(np.nan == np.nan)\n",
     "print(np.inf > np.nan)\n",
     "print(np.nan - np.nan)\n",
+	"print(np.nan in set([np.nan]))\n"
     "print(0.3 == 3 * 0.1)"
    ]
   },
@@ -435,8 +436,7 @@
    "outputs": [],
    "source": [
     "Z = np.random.random((5,5))\n",
-    "Zmax, Zmin = Z.max(), Z.min()\n",
-    "Z = (Z - Zmin)/(Zmax - Zmin)\n",
+    "Z = (Z - np.mean (Z)) / (np.std (Z))\n",
     "print(Z)"
    ]
   },
diff --git a/100_Numpy_exercises.md b/100_Numpy_exercises.md
index 8b5e393..14ae2c5 100644
--- a/100_Numpy_exercises.md
+++ b/100_Numpy_exercises.md
@@ -146,6 +146,7 @@ print(0 * np.nan)
 print(np.nan == np.nan)
 print(np.inf > np.nan)
 print(np.nan - np.nan)
+print(np.nan in set([np.nan]))
 print(0.3 == 3 * 0.1)
 ```
 
@@ -187,8 +188,7 @@ print(Z)
 
 ```python
 Z = np.random.random((5,5))
-Zmax, Zmin = Z.max(), Z.min()
-Z = (Z - Zmin)/(Zmax - Zmin)
+Z = (Z - np.mean (Z)) / (np.std (Z))
 print(Z)
 ```
 
diff --git a/100_Numpy_exercises_no_solution.ipynb b/100_Numpy_exercises_no_solution.ipynb
index d38bd4c..13fccbe 100644
--- a/100_Numpy_exercises_no_solution.ipynb
+++ b/100_Numpy_exercises_no_solution.ipynb
@@ -284,6 +284,7 @@
     "np.nan == np.nan\n",
     "np.inf > np.nan\n",
     "np.nan - np.nan\n",
+	"np.nan in set([np.nan])\n",
     "0.3 == 3 * 0.1\n",
     "```"
    ]
diff --git a/100_Numpy_exercises_no_solution.md b/100_Numpy_exercises_no_solution.md
index 6e10456..44b3fbe 100644
--- a/100_Numpy_exercises_no_solution.md
+++ b/100_Numpy_exercises_no_solution.md
@@ -82,6 +82,7 @@ free to open an issue at <https://github.com/rougier/numpy-100>
 np.nan == np.nan
 np.inf > np.nan
 np.nan - np.nan
+np.nan in set([np.nan])
 0.3 == 3 * 0.1
 ```
 
diff --git a/100_Numpy_exercises_with_hint.ipynb b/100_Numpy_exercises_with_hint.ipynb
index 5e1a70f..6361ffd 100644
--- a/100_Numpy_exercises_with_hint.ipynb
+++ b/100_Numpy_exercises_with_hint.ipynb
@@ -301,6 +301,7 @@
     "np.nan == np.nan\n",
     "np.inf > np.nan\n",
     "np.nan - np.nan\n",
+	"np.nan in set([np.nan])\n"
     "0.3 == 3 * 0.1\n",
     "```"
    ]
@@ -387,7 +388,7 @@
    "metadata": {},
    "source": [
     "#### 22. Normalize a 5x5 random matrix (★☆☆) \n",
-    "(**hint**: (x - min) / (max - min))"
+    "(**hint**: (x - mean) / std)"
    ]
   },
   {
diff --git a/100_Numpy_exercises_with_hint.md b/100_Numpy_exercises_with_hint.md
index cba1f71..43aaccb 100644
--- a/100_Numpy_exercises_with_hint.md
+++ b/100_Numpy_exercises_with_hint.md
@@ -116,6 +116,7 @@ free to open an issue at <https://github.com/rougier/numpy-100>
 np.nan == np.nan
 np.inf > np.nan
 np.nan - np.nan
+np.nan in set([np.nan])
 0.3 == 3 * 0.1
 ```
 
@@ -145,7 +146,7 @@ np.nan - np.nan
 
 #### 22. Normalize a 5x5 random matrix (★☆☆) 
 
-(**hint**: (x - min) / (max - min))
+(**hint**: (x - mean) / std)