diff --git a/pymath/__init__.py b/pymath/__init__.py
index e69de29..cbe76dd 100644
--- a/pymath/__init__.py
+++ b/pymath/__init__.py
@@ -0,0 +1,3 @@
+from pymath.arithmetic import add, subtract, multiply, divide
+
+__all__ = ["add", "subtract", "multiply", "divide"]
diff --git a/pymath/arithmetic.py b/pymath/arithmetic.py
index e69de29..9ffb8f4 100644
--- a/pymath/arithmetic.py
+++ b/pymath/arithmetic.py
@@ -0,0 +1,27 @@
+# mathlib/arithmetic.py
+
+
+def add(a: float, b: float) -> float:
+    """Return the sum of a and b."""
+    return a + b
+
+
+def subtract(a: float, b: float) -> float:
+    """Return a minus b."""
+    return a - b
+
+
+def multiply(a: float, b: float) -> float:
+    """Return a multiplied by b."""
+    return a * b
+
+
+def divide(a: float, b: float) -> float:
+    """Return a divided by b.
+
+    Raises:
+        ValueError: if b is zero.
+    """
+    if b == 0:
+        raise ValueError("Cannot divide by zero")
+    return a / b
diff --git a/pyproject.toml b/pyproject.toml
index ffd8eac..365f461 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=68"]
-build-backend = "setuptools.backends.legacy:build"
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "mathlib"
@@ -25,4 +25,4 @@ select = ["E", "F", "W"]   # pycodestyle + pyflakes rules
 testpaths = ["tests"]
 
 [tool.coverage.report]
-omit = ["tests/*"]
\ No newline at end of file
+omit = ["tests/*"]
diff --git a/test/test_arithmetic.py b/test/test_arithmetic.py
index e69de29..4848416 100644
--- a/test/test_arithmetic.py
+++ b/test/test_arithmetic.py
@@ -0,0 +1,42 @@
+# tests/test_arithmetic.py
+import pytest
+from pymath import add, subtract, multiply, divide
+
+
+class TestAdd:
+    def test_positive_numbers(self):
+        assert add(2, 3) == 5
+
+    def test_negative_numbers(self):
+        assert add(-1, -2) == -3
+
+    def test_floats(self):
+        assert add(0.1, 0.2) == pytest.approx(0.3)  # ← important!
+
+
+class TestSubtract:
+    def test_basic(self):
+        assert subtract(10, 4) == 6
+
+    def test_result_is_negative(self):
+        assert subtract(2, 5) == -3
+
+
+class TestMultiply:
+    def test_basic(self):
+        assert multiply(3, 4) == 12
+
+    def test_by_zero(self):
+        assert multiply(99, 0) == 0
+
+
+class TestDivide:
+    def test_basic(self):
+        assert divide(10, 2) == 5.0
+
+    def test_returns_float(self):
+        assert isinstance(divide(7, 2), float)
+
+    def test_divide_by_zero_raises(self):
+        with pytest.raises(ValueError, match="Cannot divide by zero"):
+            divide(5, 0)