input: Write a Python function to transform an octal number into its decimal equivalent.  
  assert octal_to_decimal(25) == 21  
  assert octal_to_decimal(30) == 24  
  assert octal_to_decimal(40) == 32
output: def octal_to_decimal(octal):
      decimal = 0
      octal_str = str(octal)
      for i in range(len(octal_str)):
          decimal += int(octal_str[-(i+1)]) * (8 ** i)
      return decimal