Teacher Liu has 51 essays to correct, and 15 of them have already been corrected. If 9 essays are corrected every hour, how many hours will it take to complete the correction?
刘老师要批改51篇作文，已经批改15篇，如果每小时批改9篇，还要几小时能够批改完？
<gadget id="calculator">51 - 15</gadget>
<output>36</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

<result>4</result>