In order to encourage residents to save water, the water company stipulates that each household will be charged 2.2 yuan per ton for water within 15 tons (including 15 tons) per month, and 3.5 yuan per ton for excess tonnage exceeding 15 tons. A total of 40 yuan was paid for water. How many tons did Liangliang’s house 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2.2元收费，超过15吨的超出的吨数按每吨3.5元收费，亮亮家上月共交水费40元，亮亮家上月用水多少吨？
<gadget id="calculator">2.2 * 15</gadget>
<output>33</output>

<gadget id="calculator">40 - 33</gadget>
<output>7</output>

<gadget id="calculator">7 / 3.5</gadget>
<output>2</output>

<gadget id="calculator">15 + 2</gadget>
<output>17</output>

<result>17</result>