Many cities in our country lack water resources. In order to strengthen residents' awareness of water conservation and rational use of water resources, many cities have formulated water charging standards. City A stipulates the standard water consumption per household per month. The part that does not exceed the standard water consumption is charged at 1.2 yuan per cubic meter, and the part that exceeds the standard water consumption is charged at 3 yuan per cubic meter. Uncle Zhang’s house in this city used 9 cubic meters of water in May, and a fee of 16.2 yuan was required. What is the standard monthly water consumption per household in city A?
我国很多城市水资源缺乏，为了加强居民的节水意识，合理利用水资源，很多城市制定了用水收费标准．A市规定了每户每月的标准用水量，不超过标准用水量的部分按每立方米1.2元收费，超过标准用水量的部分按每立方米3元收费．该市张大爷家5月份用水9立方米，需交费16.2元．A市规定的每户每月标准用水量是多少立方米？
<gadget id="calculator">27 - 16.2</gadget>
<output>10.8</output>

<gadget id="calculator">3 - 1.2</gadget>
<output>1.8</output>

<gadget id="calculator">10.8 / 1.8</gadget>
<output>6</output>

<result>6</result>