He went to bed at 9 o'clock in the evening and woke up at 6 o'clock the next morning. How many hours did he sleep in total?
明明晚上9点开始睡觉，第二日早晨6点起床，他一共睡了多少小时．
<gadget id="calculator">12 - 9</gadget>
<output>3</output>

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

<result>9</result>