diff --git a/Question_01_10/answers_cpp/answer_6.cpp b/Question_01_10/answers_cpp/answer_6.cpp
index d41e650e..543a2e17 100644
--- a/Question_01_10/answers_cpp/answer_6.cpp
+++ b/Question_01_10/answers_cpp/answer_6.cpp
@@ -6,8 +6,8 @@
 
 // Dedcrease color
 cv::Mat decrease_color(cv::Mat img){
-  int height = img.cols;
-  int width = img.rows;
+  int width = img.cols;
+  int height = img.rows;
   int channel = img.channels();
 
   cv::Mat out = cv::Mat::zeros(height, width, CV_8UC3);
diff --git a/Question_11_20/answers_cpp/answer_11.cpp b/Question_11_20/answers_cpp/answer_11.cpp
old mode 100644
new mode 100755
index 5d12deb5..8415fc3c
--- a/Question_11_20/answers_cpp/answer_11.cpp
+++ b/Question_11_20/answers_cpp/answer_11.cpp
@@ -24,20 +24,25 @@ cv::Mat mean_filter(cv::Mat img, int kernel_size){
   for (int y = 0; y < height; y++){
     for (int x = 0; x < width; x++){
       for (int c = 0; c < channel; c++){
-      v = 0;
-      
-      // get pixel sum
-      for (int dy = -pad; dy < pad + 1; dy++){
-        for (int dx = -pad; dx < pad + 1; dx++){
-          if (((y + dy) >= 0) && ((x + dx) >= 0)){
-            v += (int)img.at<cv::Vec3b>(y + dy, x + dx)[c];
-          }
-        }
-      }
+		  v = 0;
+		  count = 0;
+		  
+		  // get pixel sum
+		  for (int dy = -pad; dy < pad + 1; dy++){
+			for (int dx = -pad; dx < pad + 1; dx++){
+			  if (((y + dy) >= 0) && ((x + dx) >= 0)
+					&& ((y + dy) < height) && ((x + dx) < width)){
+				v += (int)img.at<cv::Vec3b>(y + dy, x + dx)[c];
+				count++;
+			  }
+			}
+		  }
 
-      // assign mean value
-      v /= (kernel_size * kernel_size);
-      out.at<cv::Vec3b>(y, x)[c] = (uchar)v;
+		  // assign mean value
+		  if (count > 0) {
+			  v /= count;
+			  out.at<cv::Vec3b>(y, x)[c] = (uchar)v;
+		  }
       }
     }
   }
diff --git a/Question_21_30/answers_cpp/answer_23.cpp b/Question_21_30/answers_cpp/answer_23.cpp
old mode 100644
new mode 100755
index febfa92e..e9b701b6
--- a/Question_21_30/answers_cpp/answer_23.cpp
+++ b/Question_21_30/answers_cpp/answer_23.cpp
@@ -17,14 +17,14 @@ cv::Mat histogram_equalization(cv::Mat img){
 
   // histogram equalization hyper-parameters
   double Zmax = 255;
-  double hist[255];
+  double hist[256];
   double S = height * width * channel;
   int val;
   double hist_sum = 0;
 
 
   // histogram initialization
-  for (int i = 0; i < 255; i++){
+  for (int i = 0; i < 256; i++){
      hist[i] = 0;
   }
 
@@ -46,7 +46,7 @@ cv::Mat histogram_equalization(cv::Mat img){
 
         // get histogram sum <= current pixel value
         hist_sum = 0;
-        for (int l = 0; l < val; l++){
+        for (int l = 0; l <= val; l++){
           hist_sum += hist[l];
         }
         // assign equalized value
diff --git a/Question_21_30/answers_cpp/answer_26.cpp b/Question_21_30/answers_cpp/answer_26.cpp
index 291b3c26..e64e2b52 100644
--- a/Question_21_30/answers_cpp/answer_26.cpp
+++ b/Question_21_30/answers_cpp/answer_26.cpp
@@ -38,7 +38,7 @@ cv::Mat bilinear(cv::Mat img, double rx, double ry){
         val = (1. - dx) * (1. - dy) * img.at<cv::Vec3b>(y_before, x_before)[c] +
           dx * (1. - dy) * img.at<cv::Vec3b>(y_before, x_before + 1)[c] +
           (1. - dx) * dy * img.at<cv::Vec3b>(y_before + 1, x_before)[c] +
-          dx * dy * img.at<cv::Vec3b>(y_before + 1, x_before)[c];
+          dx * dy * img.at<cv::Vec3b>(y_before + 1, x_before + 1)[c];
 
         // assign pixel to new position
         out.at<cv::Vec3b>(y, x)[c] = (uchar)val;