input: Write a python function to transform an octal number into a decimal number.
  assert octal_to_decimal(25) == 21
  assert octal_to_decimal(30) == 24
  assert octal_to_decimal(40) == 32
output: def octal_to_decimal(octal_number):
      decimal_number = 0
      base = 1
      while octal_number > 0:
          last_digit = octal_number % 10
          octal_number = octal_number // 10
          decimal_number += last_digit * base
          base *= 8
      return decimal_number