The master and apprentice plan to make 156 parts, the master makes 18 parts per hour, and the apprentice makes 12 parts per hour. After the master has done 36 tasks, how many hours will it take for the master and apprentice to work together to complete the task?
师徒两人计划做156个零件，师傅每小时做18个，徒弟每小时做12个．师傅做了36个后，师徒两人合做还要多少小时才能完成任务？
<gadget id="calculator">156 - 36</gadget>
<output>120</output>

<gadget id="calculator">18 + 12</gadget>
<output>30</output>

<gadget id="calculator">120 / 30</gadget>
<output>4</output>

<result>4</result>