diff --git a/100_Numpy_exercises.ipynb b/100_Numpy_exercises.ipynb
index bef4300..4fd1119 100644
--- a/100_Numpy_exercises.ipynb
+++ b/100_Numpy_exercises.ipynb
@@ -989,7 +989,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★★)"
+    "#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)"
    ]
   },
   {
diff --git a/100_Numpy_exercises.md b/100_Numpy_exercises.md
index 5f55706..3319bc0 100644
--- a/100_Numpy_exercises.md
+++ b/100_Numpy_exercises.md
@@ -178,7 +178,7 @@ np.sqrt(-1) == np.emath.sqrt(-1)
 
 #### 65. How to accumulate elements of a vector (X) to an array (F) based on an index list (I)? (★★★)
 
-#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★★)
+#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)
 
 #### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
 
diff --git a/100_Numpy_exercises_with_hints.md b/100_Numpy_exercises_with_hints.md
index 4a107b9..74c1bca 100644
--- a/100_Numpy_exercises_with_hints.md
+++ b/100_Numpy_exercises_with_hints.md
@@ -178,7 +178,7 @@ np.sqrt(-1) == np.emath.sqrt(-1)
 `hint: np.bincount | np.add.at`
 #### 65. How to accumulate elements of a vector (X) to an array (F) based on an index list (I)? (★★★)
 `hint: np.bincount`
-#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★★)
+#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)
 `hint: np.unique`
 #### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
 `hint: sum(axis=(-2,-1))`
diff --git a/100_Numpy_exercises_with_hints_with_solutions.md b/100_Numpy_exercises_with_hints_with_solutions.md
index bd311ce..a8eabc6 100644
--- a/100_Numpy_exercises_with_hints_with_solutions.md
+++ b/100_Numpy_exercises_with_hints_with_solutions.md
@@ -674,17 +674,17 @@ I = [1,3,9,3,4,1]
 F = np.bincount(I,X)
 print(F)
 ```
-#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★★)
+#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)
 `hint: np.unique`
 
 ```python
-# Author: Nadav Horesh
+# Author: Fisher Wang
 
-w,h = 16,16
-I = np.random.randint(0,2,(h,w,3)).astype(np.ubyte)
-F = I[...,0]*256*256 + I[...,1]*256 +I[...,2]
-n = len(np.unique(F))
-print(np.unique(I))
+w, h = 256, 256
+I = np.random.randint(0, 4, (w, h, 3)).astype(np.ubyte)
+colors = np.unique(I.reshape(-1, 3), axis=0)
+n = len(colors)
+print(n)
 ```
 #### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
 `hint: sum(axis=(-2,-1))`
diff --git a/100_Numpy_exercises_with_solutions.md b/100_Numpy_exercises_with_solutions.md
index 12c658a..4c13301 100644
--- a/100_Numpy_exercises_with_solutions.md
+++ b/100_Numpy_exercises_with_solutions.md
@@ -674,18 +674,17 @@ I = [1,3,9,3,4,1]
 F = np.bincount(I,X)
 print(F)
 ```
-#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★★)
+#### 66. Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)
 
 
 ```python
-# Author: Nadav Horesh
+# Author: Fisher Wang
 
-w,h = 16,16
-I = np.random.randint(0,2,(h,w,3)).astype(np.ubyte)
-# The parenthesis around 256*256 ensure the dtpye promoted to uint32 properly.Otherwise the first term will overflow.
-F = I[...,0]*(256*256) + I[...,1]*256 +I[...,2]
-n = len(np.unique(F))
-print(np.unique(I))
+w, h = 256, 256
+I = np.random.randint(0, 4, (w, h, 3)).astype(np.ubyte)
+colors = np.unique(I.reshape(-1, 3), axis=0)
+n = len(colors)
+print(n)
 ```
 #### 67. Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)
 
diff --git a/source/exercises100.ktx b/source/exercises100.ktx
index 5f392a7..d022bac 100644
--- a/source/exercises100.ktx
+++ b/source/exercises100.ktx
@@ -855,19 +855,19 @@ F = np.bincount(I,X)
 print(F)
 
 < q66
-Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★★)
+Considering a (w,h,3) image of (dtype=ubyte), compute the number of unique colors (★★☆)
 
 < h66
 hint: np.unique
 
 < a66
-# Author: Nadav Horesh
+# Author: Fisher Wang
 
-w,h = 16,16
-I = np.random.randint(0,2,(h,w,3)).astype(np.ubyte)
-F = I[...,0]*256*256 + I[...,1]*256 +I[...,2]
-n = len(np.unique(F))
-print(np.unique(I))
+w, h = 256, 256
+I = np.random.randint(0, 4, (w, h, 3)).astype(np.ubyte)
+colors = np.unique(I.reshape(-1, 3), axis=0)
+n = len(colors)
+print(n)
 
 < q67
 Considering a four dimensions array, how to get sum over the last two axis at once? (★★★)