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 up to 15 tons (including 15 tons) per month, and 3.5 yuan per ton for the part exceeding 15 tons. Huanhuan's family paid a water fee of 28.5 yuan last month. How many tons of water did Huanhuan's family use 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)按每吨1.2元收费，超过15吨的部分按每吨3.5元收费．欢欢家上月缴水费28.5元，欢欢家上月用水多少吨？
<gadget id="calculator">15 * 1.2</gadget>
<output>18</output>

<gadget id="calculator">28.5 - 18</gadget>
<output>10.5</output>

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

<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<result>18</result>