A parking lot charges cars for parking: 12 yuan is charged within two hours, and 1.5 yuan is charged for every additional half hour thereafter (less than half an hour is counted as half an hour). Uncle Li parks for 3 hours and 20 minutes, how much does he pay?
某停车场对小轿车停车收费情况是：时间在两小时以内收12元，以后每增加半小时(不足半小时按半小时算)收1.5元．李叔叔停车3小时20分钟，他应付多少元？
<gadget id="calculator">1.5 * 3</gadget>
<output>4.5</output>

<gadget id="calculator">4.5 + 12</gadget>
<output>16.5</output>

<result>16.5</result>