Add type hints and default args for problem 20 (#2962)

- Improved variable names
- Added type hints
- Added default argument values for validate_solutions script

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 @@ Find the sum of the digits in the number 100!
 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__":