diff --git a/src/arithmetic/arithmetic.py b/src/arithmetic/arithmetic.py
index 86f6960..4e825c2 100644
--- a/src/arithmetic/arithmetic.py
+++ b/src/arithmetic/arithmetic.py
@@ -13,7 +13,7 @@ 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 +28,12 @@ def factorial(n: int) -> int:
     Raises:
         ValueError: if n is negative
     """
-    raise NotImplementedError
+    if n < 0:
+        raise ValueError("n must be a non-negative integer")
+    if n == 0 or n == 1:
+        return 1
+    else:
+        return n * factorial(n - 1)
 
 
 def is_prime(n: int) -> bool:
@@ -43,4 +48,9 @@ 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(n**0.5) + 1):
+        if n % i == 0:
+            return False
+    return True