A community uses a stepped water price. The water fee for less than 5 tons (including 5 tons) is 2.5 yuan per ton, and the water fee for more than 5 tons is 3 yuan per ton. Aunt Li’s water fee was 33.5 yuan last month. Ton?
某小区使用阶梯式水价，5吨以内(包括5吨)每吨水费2.5元，超过5吨部分，每吨水费3元，李阿姨家上月水费33.5元，她家上月用水多少吨？
<gadget id="calculator">2.5 * 5</gadget>
<output>12.5</output>

<gadget id="calculator">33.5 - 12.5</gadget>
<output>21</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">7 + 5</gadget>
<output>12</output>

<result>12</result>