A train departs from Shanghai at 8:00 p.m. and arrives in Beijing at 5:00 p.m. the next day. How many hours does it take?
一列火车晚上8时从上海开出，第二天下午5时到达北京，路上行了几小时？
<gadget id="calculator">24 - 20</gadget>
<output>4</output>

<gadget id="calculator">12 - 0</gadget>
<output>12</output>

<gadget id="calculator">17 - 12 - 0</gadget>
<output>5</output>

<gadget id="calculator">4 + 12 + 5</gadget>
<output>21</output>

<result>21</result>