diff --git a/collaboration/learning_goals.md b/collaboration/learning_goals.md
index 878a874b5..3c9261a17 100644
--- a/collaboration/learning_goals.md
+++ b/collaboration/learning_goals.md
@@ -93,7 +93,7 @@ the challenges in this class
 
 ---
 
-### Chrismy
+### Chrismy Augustin
 
 - Gain more knowledge and hands-on experience with industry-standard tools and
 technologies like VS Code, Python, GitHub.  
diff --git a/solutions/challenge_9/__init__.py b/solutions/challenge_9/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/solutions/challenge_9/sum_of_digits.py b/solutions/challenge_9/sum_of_digits.py
new file mode 100644
index 000000000..a758eaf37
--- /dev/null
+++ b/solutions/challenge_9/sum_of_digits.py
@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+A module for calculating the sum of the digits of a positive integer.
+
+Module contents:
+    sum_of_digits: calculates the sum of the digits of a positive integer.
+
+Created on January 3rd, 2025
+@author: Semira Tesfai
+"""
+
+
+def sum_of_digits(n: int) -> int:
+    """Calculate the sum of the digits of a positive integer.
+
+    Parameters
+    ----------
+    n : int
+        The non-negative integer whose digits will be summed.
+
+    Returns
+    -------
+    int
+        The sum of the digits of the given integer.
+
+    Raises
+    ------
+    ValueError
+        If the argument is not a non-negative integer.
+
+    Examples
+    --------
+    >>> sum_of_digits(123)
+    6
+    >>> sum_of_digits(4567)
+    22
+    >>> sum_of_digits(0)
+    0
+    """
+
+    if not isinstance(n, int) or n < 0:
+        raise ValueError("input must be a non-negative integer.")
+
+    return sum(int(digit) for digit in str(n))
diff --git a/solutions/tests/challenge_9/__init__.py b/solutions/tests/challenge_9/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/solutions/tests/challenge_9/test_sum_of_digits.py b/solutions/tests/challenge_9/test_sum_of_digits.py
new file mode 100644
index 000000000..a509212cf
--- /dev/null
+++ b/solutions/tests/challenge_9/test_sum_of_digits.py
@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Unit tests for the sum_of_digits function.
+Created on January 3rd, 2025
+"""
+
+import unittest
+
+from solutions.challenge_9.sum_of_digits import sum_of_digits
+
+
+class TestSumOfDigits(unittest.TestCase):
+    """Tests for the sum_of_digits function."""
+
+    def test_with_positive_integer(self):
+        """Ensure that the sum of digits of a positive integer is calculated correctly."""
+        self.assertEqual(sum_of_digits(123), 6)
+
+    def test_with_large_integer(self):
+        """Ensure that the sum of digits of a large positive integer is calculated correctly."""
+        self.assertEqual(sum_of_digits(4567), 22)
+
+    def test_with_zero(self):
+        """Ensure that the sum of digits of zero is zero."""
+        self.assertEqual(sum_of_digits(0), 0)
+
+    def test_with_invalid_input(self):
+        """Ensure that the function raises an assertion error for invalid inputs."""
+        with self.assertRaises(ValueError):
+            sum_of_digits(-5)
+        with self.assertRaises(ValueError):
+            sum_of_digits(4.5)
+        with self.assertRaises(ValueError):
+            sum_of_digits("123")
+
+
+if __name__ == "__main__":
+    unittest.main()