In order to encourage residents to save water in a certain city, the water company stipulates that each household will be charged 3.5 yuan per ton for water within 15 tons (including 15 tons), and if it exceeds 15 tons, the excess part will be charged at 5.5 yuan per ton. 21 tons of water, how much should I pay for water?
某市为了鼓励居民节约用水，自来水公司规定，每户用水15吨以内(含15吨)按每吨3.5元收费，超过15吨的，其超过部分按每吨5.5元收费，小明家上月共用水21吨，应该交水费多少元？
<gadget id="calculator">15 * 3.5</gadget>
<output>52.5</output>

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

<gadget id="calculator">6 * 5.5</gadget>
<output>33</output>

<gadget id="calculator">52.5 + 33</gadget>
<output>85.5</output>

<result>85.5</result>