There are 10 lights in the classroom, all of which are on, and now 5 lights are turned off, how many lights are left on.
教室里有10盏灯，全部亮着，现在关掉了5盏灯，还剩多少盏灯亮着．
<gadget id="calculator">10 - 5</gadget>
<output>5</output>

<result>5</result>