In order to save water resources, a certain city has established per capita water consumption standards for citizens, and charges for excess water. The water fee per cubic meter within the specified standard is 1.40 yuan, and the water fee per cubic meter of the excess part will be charged by 100%. The three members of Uncle Li’s family used 15 cubic meters of water last month and paid a total water fee of 25.2 yuan. What is the per capita monthly water consumption standard in this city?
为了节约水资源，某市制定了市民人均用水标准，对超标部分加价收费．在规定标准内每立方米的水费是1.40元，而超标部分每立方米的水费要加收100%．李叔叔家三口人，上月用水15立方米，共缴水费25.2元，这个城市人均每月用水标准是多少立方米？
<gadget id="calculator">1.4 * 15</gadget>
<output>21</output>

<gadget id="calculator">25.2 - 21</gadget>
<output>4.2</output>

<gadget id="calculator">4.2 / 1.4</gadget>
<output>3</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<result>4</result>