The tap water charges in a certain city are stipulated in this way: each household uses 15 tons of water per month (including 15 tons) and charges 1.8 yuan per ton, and those that exceed 15 tons. The excess part is charged at 3 yuan per ton. Fang Hua's family paid a water fee of 45 yuan in September. How much water did his family use in September?
某城市自来水收费是这样规定的：每户每月用水15T以内(含15T)按每吨1.8元收费，超过15T的．超过部分按每吨3元收费．方华家九月份交了45元水费，他家九月份用水多少T？
<gadget id="calculator">15 * 1.8</gadget>
<output>27</output>

<gadget id="calculator">45 - 27</gadget>
<output>18</output>

<gadget id="calculator">18 / 3</gadget>
<output>6</output>

<gadget id="calculator">6 + 15</gadget>
<output>21</output>

<result>21</result>