Master Zhang processes 108 parts in 4 hours, and Master Li processes 126 parts in 6 hours. How many more parts per hour does Master Zhang process than Master Li?
张师傅4小时加工108个零件，李师傅6小时加工126个零件，张师傅比李师傅平均每小时多加工多少零件？
<gadget id="calculator">108 / 4</gadget>
<output>27</output>

<gadget id="calculator">126 / 6</gadget>
<output>21</output>

<gadget id="calculator">27 - 21</gadget>
<output>6</output>

<result>6</result>