In order to encourage residents to save water, the water company stipulates that each household will be charged 1.2 yuan per ton for water within 15 tons (including 15 tons) per month, and if it exceeds 15 tons, the excess tonnage will be charged at 5 yuan per ton. A total of 38 yuan to pay the water fee. Do you know how many tons of water Mingming's family used last month?
为鼓励居民节约用水，自来水公司规定：每户每月用水15吨以内(含15吨)按1.2元一吨收费，超过15吨的，其超出吨数按5元一吨收费，明明家上月共交水费38元．你知道明明家上月用水多少吨吗？
<gadget id="calculator">15 * 1.2</gadget>
<output>18</output>

<gadget id="calculator">38 - 18</gadget>
<output>20</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

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

<result>19</result>