In order to carry forward the spirit of "saving water", the natural water company stipulates that each household uses less than 15 tons of water per month (including 15 tons), and pays 1.2 yuan per ton, and pays 3 yuan per ton for more than 15 tons. The little party family paid 30 yuan last month. How many tons of water did their family use last month?
为弘扬"节约用水"精神，自然水公司规定：每家每月用15吨以内水(包括15吨)，每吨交1.2元，超过15吨的每吨交3元．小党家上个月交了30元，他们家上个月用了多少吨水？
<gadget id="calculator">1.2 * 15</gadget>
<output>18</output>

<gadget id="calculator">30 - 18</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">4 + 15</gadget>
<output>19</output>

<result>19</result>