diff --git a/src/math_utils.py b/src/math_utils.py
index e52f339..aca1dad 100644
--- a/src/math_utils.py
+++ b/src/math_utils.py
@@ -1,11 +1,11 @@
-"""
-Math utility functions
-"""
-
-def add(a, b):
-    """Add two numbers"""
-    return a + b
-
-def multiply(a, b):
-    """Multiply two numbers"""
-    return a * b
+def fibonacci(n):
+    if n < 0:
+        raise ValueError("n must be non-negative")
+    if n == 0:
+        return 0
+    if n == 1:
+        return 1
+    a, b = 0, 1
+    for _ in range(2, n + 1):
+        a, b = b, a + b
+    return b
diff --git a/tests/test_math_utils.py b/tests/test_math_utils.py
index 516f210..a16091f 100644
--- a/tests/test_math_utils.py
+++ b/tests/test_math_utils.py
@@ -1,15 +1,25 @@
-import sys
-from pathlib import Path
-sys.path.insert(0, str(Path(__file__).parent.parent / 'src'))
+import pytest
+from src.math_utils import fibonacci
 
-from math_utils import add, multiply
 
-def test_add():
-    assert add(2, 3) == 5
-    assert add(-1, 1) == 0
-    assert add(0, 0) == 0
+def test_fibonacci_base_cases():
+    assert fibonacci(0) == 0
+    assert fibonacci(1) == 1
 
-def test_multiply():
-    assert multiply(2, 3) == 6
-    assert multiply(-1, 5) == -5
-    assert multiply(0, 10) == 0
+
+def test_fibonacci_small_values():
+    assert fibonacci(2) == 1
+    assert fibonacci(3) == 2
+    assert fibonacci(4) == 3
+    assert fibonacci(5) == 5
+    assert fibonacci(10) == 55
+
+
+def test_fibonacci_negative_raises():
+    with pytest.raises(ValueError) as excinfo:
+        fibonacci(-1)
+    assert str(excinfo.value) == "n must be non-negative"
+
+
+def test_fibonacci_large_value():
+    assert fibonacci(20) == 6765