In order to save water, a certain place stipulates that the water consumption is less than 20 tons, and the water price per ton is 1.5 yuan; for the part exceeding 20 tons, the water price is 2.1 yuan per ton. Xiaoming's family paid 61.5 yuan for water last month. How many tons of water was used last month?
为节约用水，某地规定用水量在20吨以内，每吨水价1.5元；超过20吨的部分，每吨水价2.1元．小明家上个月缴水费61.5元，上个月用水多少吨？
<gadget id="calculator">20 * 1.5</gadget>
<output>30</output>

<gadget id="calculator">61.5 - 30</gadget>
<output>31.5</output>

<gadget id="calculator">31.5 / 2.1</gadget>
<output>15</output>

<gadget id="calculator">15 + 20</gadget>
<output>35</output>

<result>35</result>