In order to encourage residents to save water, the waterworks company stipulates that each household will be charged 1.8 yuan per ton for water within 15 tons (including 15 tons); if it exceeds 15 tons, the excess part will be charged at 2.5 yuan per ton. The owner shared 21 tons of water last month, how much should he pay for the water?
为了鼓励居民节约用水，自来水厂公司规定：每户用水15吨以内(含15吨)按每吨1.8吨元收费；超过15吨的，其超过部分按每吨2.5元收费．小东家上月共用水21吨，应该交水费多少元？
<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">6 * 2.5</gadget>
<output>15</output>

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

<gadget id="calculator">15 + 27</gadget>
<output>42</output>

<result>42</result>