diff --git a/README.md b/README.md
index af2eb1b..74aee94 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,9 @@
 1. Fork this repository to your own GitHub account.
 2. Create a new branch from `develop` and name it `feature/student1name_student2name`.
 3. Clone the repo to AWS VM.
-4. Implement the functions until all `pytest` tests pass.
-5. Push your code to your Github repo.
-5. Submit a Pull Request back to the `develop` branch of the main repo. CI must pass.
-6. Wait for code review and grading.
+4. Run poetry install to install dependencies.
+5. Run poetry run pre-commit install to install the pre-commit hook.
+6. Implement the functions until all `pytest` tests pass. Commit and pass pre-commit checks.
+7. Push your code to your Github repo.
+8. Submit a Pull Request back to the `develop` branch of the main repo. CI must pass.
+9. Wait for code review and grading.
diff --git a/src/arithmetic/arithmetic.py b/src/arithmetic/arithmetic.py
index 86f6960..e150edd 100644
--- a/src/arithmetic/arithmetic.py
+++ b/src/arithmetic/arithmetic.py
@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import math
+
 
 def add_numbers(a: int, b: int) -> int:
     """Return the sum of two integers.
@@ -13,7 +15,7 @@ def add_numbers(a: int, b: int) -> int:
     Returns:
         Sum of a and b.
     """
-    raise NotImplementedError
+    return a + b
 
 
 def factorial(n: int) -> int:
@@ -28,7 +30,9 @@ def factorial(n: int) -> int:
     Raises:
         ValueError: if n is negative
     """
-    raise NotImplementedError
+    if n < 0:
+        raise ValueError()
+    return math.factorial(n)
 
 
 def is_prime(n: int) -> bool:
@@ -43,4 +47,9 @@ def is_prime(n: int) -> bool:
     Returns:
         True if n is prime; otherwise False.
     """
-    raise NotImplementedError
+    if n <= 1:
+        return False
+    for i in range(2, int(math.sqrt(n)) + 1):
+        if n % i == 0:
+            return False
+    return True