The master and the apprentice processed a batch of parts at the same time. They processed 21 parts in 1.5 hours, and then they processed 9 hours at the same time. At this time, the master processed 42 more parts than the apprentice. Ask the master how many parts are processed per hour.
师徒两人同时加工一批零件，1.5小时两人共加工了21个，接着两人又同时加工了9小时，这时师傅比徒弟一共多加工了42个．问师傅每小时加工多少个零件．
<gadget id="calculator">21 / 1.5</gadget>
<output>14</output>

<gadget id="calculator">9 + 1.5</gadget>
<output>10.5</output>

<gadget id="calculator">42 / 10.5</gadget>
<output>4</output>

<gadget id="calculator">14 + 4</gadget>
<output>18</output>

<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<result>9</result>