Moved more problems to Python.

python/e015.py

@@ -0,0 +1,21 @@
+from math import factorial
+
+
+def get_number_of_routes(n):
+    """
+    There are n down and n right moves which means 2 * n moves
+    in total. All permutations are calculated with (2n)!. Next redundant
+    moves have to be canceled out. The combinations for only down moves
+    or only right moves would (if they were different symbols) be n!.
+    Hence, the denominator is (n!)^2.
+    """
+    return factorial(2 * n) // (factorial(n) * factorial(n))
+
+
+def euler_015():
+    return get_number_of_routes(20)
+
+
+assert(get_number_of_routes(2) == 6)
+assert(euler_015() == 137846528820)
+print("e015.py: {}".format(euler_015()))