In order to save water in a certain city, the tap water fee is stipulated as follows: each household uses less than 15 tons (including 15 tons) of water per month, and is charged at 2.9 yuan/ton; if it exceeds 15 tons, the excess ton is charged at 5 yuan/ton ． How much water fee should a household pay for 21 tons of water in April?
某城市为了节约用水，自来水水费是这样规定的：每户每月用水15吨以内(含15吨)，按2.9元/吨收费，超出15吨的，其超出吨数按5元/吨收费．某户四月份用水21吨，应交多少元水费？
<gadget id="calculator">15 * 2.9</gadget>
<output>43.5</output>

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

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">43.5 + 30</gadget>
<output>73.5</output>

<result>73.5</result>