Ming Ming gets up at 6 o'clock in the morning, in order to ensure 9 hours of sleep time, how long should he go to bed on the first night?
明明早上6时起床，为保证9个小时的睡觉时间，第一天晚上他应在多少时前睡觉．
<gadget id="calculator">9 - 6 - 0</gadget>
<output>3</output>

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>