The master and apprentice processed 198 parts at the same time, and the plan was to complete in 6 hours. The apprentice processed 15 parts per hour. How many parts should the master process at least per hour to complete the task on time?
师徒二人同时加工零件198个，计划6小时完成，徒弟每小时加工15个，师傅每小时至少加工多少个零件才能按时完成任务？
<gadget id="calculator">198 / 6</gadget>
<output>33</output>

<gadget id="calculator">33 - 15</gadget>
<output>18</output>

<result>18</result>