Aunt Li's water bill this month is 28.5 yuan. How many tons of water did her family use this month? Announcement on Water Fee Adjustment Each household uses less than 15 tons of water per month (including 15 tons), and the fee is 1.20 yuan per ton; more than 15 tons, the excess part is charged 3.00 yuan per ton.
李阿姨家这个月的水费是28.5元．她家这个月用了多少吨水？水费调整公告每户每月用水15吨以下(含15吨)，按每吨1.20元收费；超过15吨，超出的部分每吨3.00元收费．
<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</gadget>
<output>3.5</output>

<gadget id="calculator">15 + 3.5</gadget>
<output>18.5</output>

<result>18.5</result>