diff --git a/basics/variable.py b/basics/variable.py
new file mode 100644
index 0000000..10fc208
--- /dev/null
+++ b/basics/variable.py
@@ -0,0 +1,63 @@
+def main():
+    """
+    >>> a = 3
+    >>> b = 4
+    >>> a + b
+    7
+    >>> a - b
+    -1
+    >>> a * b
+    12
+    >>> a / b
+    0.75
+    >>> a // b
+    0
+    >>> a % b
+    3
+    >>> 3 ** 4
+    81
+
+    >>> type(3)
+    <class 'int'>
+    >>> type(3.14)
+    <class 'float'>
+    >>> type('a')
+    <class 'str'>
+    >>> type("abc")
+    <class 'str'>
+    >>> type(True)
+    <class 'bool'>
+    >>> type(None)
+    <class 'NoneType'>
+    >>> type(3 + 4j)
+    <class 'complex'>
+
+    >>> int(3.14)
+    3
+    >>> int(-3)
+    -3
+    >>> float("3.14")
+    3.14
+    >>> str(3.14)
+    '3.14'
+    >>> str(3 + 4j)
+    '(3+4j)'
+    >>> chr(65)
+    'A'
+    >>> chr(97)
+    'a'
+    >>> ord("a")
+    97
+    >>> ord("A")
+    65
+    >>> chr(ord('a') - 32)
+    'A'
+    >>> chr(ord('A') + 32)
+    'a'
+    """
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
diff --git a/conversions/__init__.py b/conversions/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/conversions/fahrenheit_to_celsius.py b/conversions/fahrenheit_to_celsius.py
new file mode 100644
index 0000000..ea342d3
--- /dev/null
+++ b/conversions/fahrenheit_to_celsius.py
@@ -0,0 +1,19 @@
+"""
+https://en.wikipedia.org/wiki/Fahrenheit
+"""
+
+
+def fahrenheit_to_celsius(temperature: float) -> float:
+    """
+    >>> fahrenheit_to_celsius(32)
+    0.0
+    >>> fahrenheit_to_celsius(39)
+    3.888888888888889
+    """
+    return 5 * (temperature - 32) / 9
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
diff --git a/maths/median.py b/maths/median.py
index d34144f..f4681e0 100644
--- a/maths/median.py
+++ b/maths/median.py
@@ -22,7 +22,8 @@ def median(numbers):
     numbers = sorted(numbers)
     mid_index = len(numbers) // 2
     return (
-        (numbers[mid_index] + numbers[mid_index - 1]) / 2 if mid_index % 2 == 0
+        (numbers[mid_index] + numbers[mid_index - 1]) / 2
+        if mid_index % 2 == 0
         else numbers[mid_index]
     )
 
diff --git a/maths/mode.py b/maths/mode.py
new file mode 100644
index 0000000..f4df405
--- /dev/null
+++ b/maths/mode.py
@@ -0,0 +1,33 @@
+"""
+https://en.wikipedia.org/wiki/Mode_(statistics)
+"""
+
+
+def mode(numbers):
+    """
+    Calculate mode of a list numbers.
+    :param numbers: the numbers
+    :return: mode number of the numbers.
+
+    >>> mode([1, 2, 2, 3, 4, 7, 9])
+    2
+    """
+    max_count = 1
+    mode_number = numbers[0]
+
+    for number in numbers:
+        count = 0
+        for temp in numbers:
+            if temp == number:
+                count += 1
+        if count > max_count:
+            max_count = count
+            mode_number = number
+
+    return mode_number
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()