The tap water charges in a certain city are stipulated as follows: each household uses less than 15 tons of water per month (including 15 tons), 2.4 yuan per ton, and the excess tonnage is charged at 3 yuan per ton. How much water fee should a household pay for 21 tons of water in April?
某城市自来水收费是这样规定的：每户每月用水15吨以内(含15吨)，每吨2.4元，超出吨数按3元一吨收费．某户四月份用水21吨，应交多少元水费？
<gadget id="calculator">15 * 2.4</gadget>
<output>36</output>

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

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

<gadget id="calculator">36 + 18</gadget>
<output>54</output>

<result>54</result>