Uncle Li works the night shift at 21:00 and leaves work at 6:00 the next morning. How many hours of night shift did he work?
李叔叔21时上夜班，第二天早上6时下班．他上了多少小时的夜班．
<gadget id="calculator">24 - 21</gadget>
<output>3</output>

<gadget id="calculator">3 + 6</gadget>
<output>9</output>

<result>9</result>