code: public function of($number)
      {
          $value = clone $this;
          $value->bcmul($number)->bcdiv(100);
          if ($number instanceof Number) {
              $class = get_class($number);
              $value = new $class($value);
          } else {
              if (is_numeric($number)) {
                  $value = new Number($value);
              } else {
                  throw new \InvalidArgumentException();
              }
          }
          return $value;
      }
docs: Percentage OF $number is how much Number?
  
  @param Number|String $number
  @return Number
  Depending on the input, but always an object extended of Number
  
  @throws \InvalidArgumentException