parking lot stipulates that a parking fee of 5 yuan is charged within 2 hours, and the excess part is charged at 3.5 yuan per hour. Aunt Liu paid a parking fee of 15.5 yuan when she drove away. How long did she park in this parking lot?
一停车场规定2小时内收5元停车费，超过的部分按每小时3.5元收取．刘阿姨开车离开时交了15.5元的停车费，她在这个停车场停车多长时间？
<gadget id="calculator">15.5 - 5</gadget>
<output>10.5</output>

<gadget id="calculator">10.5 / 3.5</gadget>
<output>3</output>

<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<result>5</result>