Xiaofang went to bed at 21:00 and woke up at 7:00 the next morning. How many hours did Xiaofang sleep in total?
小芳从21时开始睡觉，到第二天早上7时起床，小芳一共睡了多少小时．
<gadget id="calculator">24 - 21</gadget>
<output>3</output>

<gadget id="calculator">3 + 7</gadget>
<output>10</output>

<result>10</result>