A certain city’s tap water charges stipulate that each household’s monthly water consumption within 15 tons (including 15 tons) will be charged at 0.9 yuan per ton; if it exceeds 15 tons, the excess tons will be charged at 3 yuan per ton. How much water should a household pay for 21 tons of water in April?
某城市自来水收费规定：每户每月用水15吨以内(含15吨)，按每吨0.9元收费；超过15吨，超出吨数按每吨3元收费．某户四月份用水21吨，应交多少元水费．
<gadget id="calculator">15 * 0.9</gadget>
<output>13.5</output>

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

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

<gadget id="calculator">13.5 + 18</gadget>
<output>31.5</output>

<result>31.5</result>