Xiao Ming goes to bed at 9 o'clock every night and wakes up at 6 o'clock in the morning. How many hours does he sleep every day?
小明每天晚上9点睡觉，早晨6点起床，他每天睡多少小时．
<gadget id="calculator">24 - 21</gadget>
<output>3</output>

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

<result>9</result>