Mr. Zhang has to correct 58 math test papers, 18 of which have been corrected in the morning, and will be corrected for 2 hours in the afternoon.
张老师要批改58份数学试卷，上午已经批改了18份，下午还要批改2小时，下午平均每小时要批改多少份？
<gadget id="calculator">58 - 18</gadget>
<output>40</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<result>20</result>