A certain city has formulated a water standard for residents, and the price of the part exceeding the standard will be increased. If the water fee per cubic meter within the standard water volume is 1.4 yuan, the water fee per cubic meter exceeding the standard will increase by 100%. There are three people in Xiaoming's family. In May, they used 15 cubic meters of water and paid a water fee of 25.2 yuan. How many cubic meters is the maximum standard of water consumption per month for a family of three in a certain city?
某城市制定了居民用水标准，超标部分加价收费．如果在标准水量内每立方米的水费是1.4元，超标部分每立方米的水费增加100%．小明家有三口人，五月份用水15立方米，交水费25.2元．某城市三口之家每月用水量最高标准规定为多少立方米？
<gadget id="calculator">1.4 * 15</gadget>
<output>21</output>

<gadget id="calculator">100 / 100</gadget>
<output>1</output>

<gadget id="calculator">1.4 * 1 * 15</gadget>
<output>21</output>

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

<gadget id="calculator">21 + (-4.2)</gadget>
<output>16.8</output>

<gadget id="calculator">1.4 * 1</gadget>
<output>1.4</output>

<gadget id="calculator">1.4 - 1.4</gadget>
<output>0</output>

<gadget id="calculator">1.4 + 0</gadget>
<output>1.4</output>

<gadget id="calculator">16.8 / 1.4</gadget>
<output>12</output>

<result>12</result>