In order to encourage residents to save water, the water company stipulates that each household will be charged 1.5 yuan per ton for water within 12 tons (including 12 tons) per month, and 4 yuan per ton for excess tonnage exceeding 12 tons. The total water fee is 40 yuan. How many tons did Xiaoqing'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12吨以内(含12吨)按每吨1.5元收费，超过12吨的超出的吨数按每吨4元收费，小青家上月共交水费40元，小青家上月用水多少吨？
<gadget id="calculator">1.5 * 12</gadget>
<output>18</output>

<gadget id="calculator">40 - 18</gadget>
<output>22</output>

<gadget id="calculator">22 / 4</gadget>
<output>11/2 = around 5.5</output>

<gadget id="calculator">12 + (11/2)</gadget>
<output>35/2 = around 17.5</output>

<result>35/2 = around 17.5</result>