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 5 yuan per ton for the excess of 15 tons. Xiaoying's family paid a total of 20.5 yuan for water last month. How many tons of water did her 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吨的其超出部分按每吨5元收费．小英家上月共缴水费20.5元，她家上月用水多少吨？
<gadget id="calculator">1.2 * 15</gadget>
<output>18</output>

<gadget id="calculator">20.5 - 18</gadget>
<output>2.5</output>

<gadget id="calculator">2.5 / 5</gadget>
<output>0.5</output>

<gadget id="calculator">0.5 + 15</gadget>
<output>15.5</output>

<result>15.5</result>