Both master and apprentice process the same kind of parts. The apprentice processes 12 pieces per hour. After working for 3 hours, the master starts to work. After 6 hours, both of them process the same number of parts. How many parts does the master process per hour?
师徒俩加工同一种零件，徒弟每小时加工12个，工作了3小时后，师傅开始工作，6小时后，两人加工的零件同样多，师傅每小时加工多少个零件？
<gadget id="calculator">3 + 6</gadget>
<output>9</output>

<gadget id="calculator">9 / 6</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">12 * (3/2)</gadget>
<output>18</output>

<result>18</result>