The charging standard of a water supply company in a certain city is as follows: each household is limited to 8 tons of water per month, and the fee is 1.8 yuan per ton, and the excess is 3.5 yuan per ton when it exceeds 8 tons. Xiaoyuan's family pays 30.15 yuan for water this month. How many tons of water does his family use this month?
某市自来水公司收费标准如下：每户每月限用水8吨，每吨收费1.8元，超过8吨时超出部分每吨收费3.5元．小元家本月缴水费30.15元．他家这个月用水多少吨？
<gadget id="calculator">8 * 1.8</gadget>
<output>14.4</output>

<gadget id="calculator">30.15 - 14.4</gadget>
<output>15.75</output>

<gadget id="calculator">15.75 / 3.5</gadget>
<output>4.5</output>

<gadget id="calculator">4.5 + 8</gadget>
<output>12.5</output>

<result>12.5</result>