diff --git a/test_utils.py b/test_utils.py
index 5a264b2..dd29733 100644
--- a/test_utils.py
+++ b/test_utils.py
@@ -30,3 +30,23 @@ def test_multiply(a, b, expected):
 def test_divide(a, b, expected):
     result = utils.divide(a, b)
     assert result == expected
+
+
+@pytest.mark.parametrize(
+    "n, expected",
+    [(0, "0b0"), (1, "0b1"), (2, "0b10"), (10, "0b1010"), (100, "0b1100100")],
+)
+def test_to_binary_valid(n, expected):
+    assert utils.to_binary(n) == expected
+
+
+@pytest.mark.parametrize("n", [-1, 101, 999])
+def test_to_binary_out_of_range(n):
+    with pytest.raises(ValueError):
+        utils.to_binary(n)
+
+
+@pytest.mark.parametrize("n", [1.5, "10", None])
+def test_to_binary_not_natural(n):
+    with pytest.raises(TypeError):
+        utils.to_binary(n)
diff --git a/utils.py b/utils.py
index 4a1de3e..11b8ffb 100644
--- a/utils.py
+++ b/utils.py
@@ -21,3 +21,12 @@ def multiply(a: int, b: int) -> int:
 def divide(a: int, b: int) -> float:
     """Divide the first integer by the second and return the result as a float."""
     return a / b
+
+
+def to_binary(n: int) -> str:
+    """Convert a natural number (0–100) to binary string. Raise if invalid."""
+    if not isinstance(n, int):
+        raise TypeError("Input must be an integer.")
+    if not 0 <= n <= 100:
+        raise ValueError("Input must be in range 0 to 100.")
+    return bin(n)