6 kyu A Rule of Divisibility by 13.py

array = [1, 10, 9, 12, 3, 4]

def thirt(n):
    total = sum([int(c) * array[i % 6] for i, c in enumerate(reversed(str(n)))])
    if n == total:
        return total
    return thirt(total)


print(thirt(8529), 79)
print(thirt(85299258), 31)
print(thirt(5634), 57)
print(thirt(1111111111), 71)
print(thirt(987654321), 30)