public function priceNetto($brutto, $tax)
    {
        $tax = round((double) $tax / 100.0, 2);

        if ($tax < 0.00) {
            throw new Exception(sprintf('Tax must be greater than or equal to 0, given %s.', $tax));
        }

        if ($tax === 0.00) {
            return $brutto;
        }

        $result = $brutto / ($tax + 1);
        
        return round($result, 2, PHP_ROUND_HALF_UP);
    }