Xiao Ming went to bed at 21:00, and got up at 6:00 the next morning. How many hours did Xiao Ming sleep in total?
小明21：00时睡觉，第二天早上6时起床，小明共睡了多少小时．
<gadget id="calculator">24 - 21</gadget>
<output>3</output>

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

<result>9</result>