input: Write a function to find the sum of all divisors of a number.
  assert sum_divisors(6) == 12
  assert sum_divisors(10) == 18
  assert sum_divisors(15) == 24
output: def sum_divisors(n):
      total = 0
      for i in range(1, n + 1):
          if n % i == 0:
              total += i
      return total