diff --git a/project_euler/problem_20/sol1.py b/project_euler/problem_20/sol1.py
index 13b3c987f046..b472024e54c0 100644
--- a/project_euler/problem_20/sol1.py
+++ b/project_euler/problem_20/sol1.py
@@ -1,4 +1,6 @@
 """
+Problem 20: https://projecteuler.net/problem=20
+
 n! means n × (n − 1) × ... × 3 × 2 × 1
 
 For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
@@ -8,14 +10,15 @@
 """
 
 
-def factorial(n):
+def factorial(num: int) -> int:
+    """Find the factorial of a given number n"""
     fact = 1
-    for i in range(1, n + 1):
+    for i in range(1, num + 1):
         fact *= i
     return fact
 
 
-def split_and_add(number):
+def split_and_add(number: int) -> int:
     """Split number digits and add them."""
     sum_of_digits = 0
     while number > 0:
@@ -25,8 +28,8 @@ def split_and_add(number):
     return sum_of_digits
 
 
-def solution(n):
-    """Returns the sum of the digits in the number 100!
+def solution(num: int = 100) -> int:
+    """Returns the sum of the digits in the factorial of num
     >>> solution(100)
     648
     >>> solution(50)
@@ -42,8 +45,8 @@ def solution(n):
     >>> solution(1)
     1
     """
-    f = factorial(n)
-    result = split_and_add(f)
+    nfact = factorial(num)
+    result = split_and_add(nfact)
     return result
 
 
diff --git a/project_euler/problem_20/sol2.py b/project_euler/problem_20/sol2.py
index 14e591795292..92e1e724a647 100644
--- a/project_euler/problem_20/sol2.py
+++ b/project_euler/problem_20/sol2.py
@@ -1,4 +1,6 @@
 """
+Problem 20: https://projecteuler.net/problem=20
+
 n! means n × (n − 1) × ... × 3 × 2 × 1
 
 For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
@@ -9,8 +11,8 @@
 from math import factorial
 
 
-def solution(n):
-    """Returns the sum of the digits in the number 100!
+def solution(num: int = 100) -> int:
+    """Returns the sum of the digits in the factorial of num
     >>> solution(100)
     648
     >>> solution(50)
@@ -26,7 +28,7 @@ def solution(n):
     >>> solution(1)
     1
     """
-    return sum([int(x) for x in str(factorial(n))])
+    return sum([int(x) for x in str(factorial(num))])
 
 
 if __name__ == "__main__":
diff --git a/project_euler/problem_20/sol3.py b/project_euler/problem_20/sol3.py
index 13f9d7831c47..4f28ac5fcfde 100644
--- a/project_euler/problem_20/sol3.py
+++ b/project_euler/problem_20/sol3.py
@@ -1,4 +1,6 @@
 """
+Problem 20: https://projecteuler.net/problem=20
+
 n! means n × (n − 1) × ... × 3 × 2 × 1
 
 For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
@@ -9,8 +11,8 @@
 from math import factorial
 
 
-def solution(n):
-    """Returns the sum of the digits in the number 100!
+def solution(num: int = 100) -> int:
+    """Returns the sum of the digits in the factorial of num
     >>> solution(1000)
     10539
     >>> solution(200)
@@ -32,7 +34,7 @@ def solution(n):
     >>> solution(0)
     1
     """
-    return sum(map(int, str(factorial(n))))
+    return sum(map(int, str(factorial(num))))
 
 
 if __name__ == "__main__":
diff --git a/project_euler/problem_20/sol4.py b/project_euler/problem_20/sol4.py
index 4c597220f09b..b32ce309dfa6 100644
--- a/project_euler/problem_20/sol4.py
+++ b/project_euler/problem_20/sol4.py
@@ -1,4 +1,6 @@
 """
+Problem 20: https://projecteuler.net/problem=20
+
 n! means n × (n − 1) × ... × 3 × 2 × 1
 
 For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
@@ -8,8 +10,8 @@
 """
 
 
-def solution(n):
-    """Returns the sum of the digits in the number 100!
+def solution(num: int = 100) -> int:
+    """Returns the sum of the digits in the factorial of num
     >>> solution(100)
     648
     >>> solution(50)
@@ -27,7 +29,7 @@ def solution(n):
     """
     fact = 1
     result = 0
-    for i in range(1, n + 1):
+    for i in range(1, num + 1):
         fact *= i
 
     for j in str(fact):