diff --git a/s1_a4.py b/s1_a4.py
index 2f2e68d..2cecd1c 100644
--- a/s1_a4.py
+++ b/s1_a4.py
@@ -5,6 +5,6 @@ if __name__ == "__main__":
     approximate_value = 0
     divider = 1
     while series_value != round(approximate_value, 6):
-        approximate_value += 1/divider**2
+        approximate_value = 1/divider**2
         divider += 1
     print("The sum equals", series_value, "after adding", divider-1, "reciprocal square numbers.")
diff --git a/s1_a51.py b/s1_a51.py
index 48b8730..bcd0f2c 100644
--- a/s1_a51.py
+++ b/s1_a51.py
@@ -3,17 +3,17 @@ class SparseMatrix:
         self.__matrix = dict()
 
     def save(self, matrix) -> None:
-        for row in range(len(matrix)):
-            for col in range(len(matrix[row])):
-                if matrix[row][col] != 0:
-                    self.__matrix[(row, col)] = matrix[row][col]
+        for row, values in enumerate(matrix):
+            for col, value in enumerate(values):
+                if value != 0:
+                    self.__matrix[(row, col)] = value
 
     def get(self, row, col) -> int or None:
         if self.__matrix[(row, col)]:
             return self.__matrix[(row, col)]
 
     def __str__(self) -> str:
-        return '\n'.join(map(lambda pair: ': '.join(map(str, pair)), self.__matrix.items()))
+        return '\n'.join(map(lambda pair: ' : '.join(map(str, pair)), self.__matrix.items()))
 
 
 if __name__ == "__main__":
diff --git a/s1_a53_6.py b/s1_a53_6.py
index 1b70a21..118843f 100644
--- a/s1_a53_6.py
+++ b/s1_a53_6.py
@@ -5,8 +5,7 @@ if __name__ == "__main__":
 
     # 5.4
     def remove_duplicates(word: str):
-        result = "".join(sorted(set(word), key=word.index))
-        print(word, len(word), "\n", result, len(result))
+        return "".join(sorted(set(word), key=word.index))
     remove_duplicates("Donaudampfschiffahrtsgesellschaftsstewardess")
 
     # 5.5
@@ -15,5 +14,4 @@ if __name__ == "__main__":
 
     # 5.6
     def get_animal_sound(animal: str) -> str:
-        animal_dictionary = {"Lion": "Growl", "Dog": "Bark", "Cat": "Meow"}
-        return animal_dictionary.get(animal, "Not found.")
+        return {"Lion": "Growl", "Dog": "Bark", "Cat": "Meow"}.get(animal, "Not found.")