In order to encourage residents to save water, the water company stipulates that each household’s monthly water consumption is within 15 tons (including 15 tons), and the fee is 1.8 yuan per ton. Last month, we shared 21 tons of water. How much should we pay for water?
为了鼓励居民节约用水，自来水公司规定每户每月用水在15吨以内(含15吨)，按每吨1.8元收费，超过15吨的，其超出部分(按每吨3元收费，)小明家上月共用水21吨，应该交水费多少元？
<gadget id="calculator">21 - 15</gadget>
<output>6</output>

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

<gadget id="calculator">1.8 * 15</gadget>
<output>27</output>

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

<result>45</result>