The charging regulations for tap water in a certain urban area are as follows: each household uses less than 15 tons (including 15 tons) of water per month, and the fee is 2.5 yuan per ton. Pay the water fee 43.5 yuan, how many tons of water?
某市区自来水收费规定如下：每户每月用水15吨以内(含15吨)，按每吨2.5元收费，超过15吨的，超过部分每吨收费比原来多收0.5元，小明家12月份交水费43.5元，用水多少吨？
<gadget id="calculator">2.5 * 15</gadget>
<output>37.5</output>

<gadget id="calculator">43.5 - 37.5</gadget>
<output>6</output>

<gadget id="calculator">2.5 + 0.5</gadget>
<output>3</output>

<gadget id="calculator">6 / 3</gadget>
<output>2</output>

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

<result>17</result>