public function compute_final_grade($responses, $totaltries) {
        $fractionsum = 0;
        $fractionmax = 0;
        foreach ($this->subquestions as $i => $subq) {
            $fractionmax += $subq->defaultmark;

            $subqa = new question_attempt($subq, 0, null, $subq->defaultmark);
            $initdata = array(); // TODO need to get this from somewhere, but I can't think where.
            $initdata['_triesleft'] = count($this->hints) + 1;
            $subqa->start('interactive', 0, $initdata);

            foreach ($responses as $response) {
                $substep = $this->get_substep(null, $i);
                $subresp = $substep->filter_array($response);
                $subresp['-submit'] = 1;
                $subqa->process_action($subresp);
                // This will fail if a student submits 'right', 'wrong', 'right' to one subquestion.
            }

            if (!$subqa->get_state()->is_finished()) {
                $subqa->finish();
            }

            $fractionsum += $subqa->get_mark();
        }
        return $fractionsum / $fractionmax;
    }