In order to encourage residents to save water, the water supply company stipulates that when the water used by each person does not exceed 2 cubic meters per month, the fee is 2.5 yuan per cubic meter; the part exceeding 2 cubic meters is charged at 4 yuan per cubic meter. There are 3 members of Xiaohua's family, and they paid a total of 39 yuan for water last month. How many cubic meters of water did Xiaohua's house use last month?
供水公司为了鼓励居民节约用水，规定每人每月用水不超过2立方米时，按每立方米2.5元收费；超过2立方米的部分按每立方米4元收费．小华家3口人，上个月共缴水费39元．小华家上个月用水多少立方米？
<gadget id="calculator">2 * 3 * 2.5</gadget>
<output>15</output>

<gadget id="calculator">39 - 15</gadget>
<output>24</output>

<gadget id="calculator">24 / 4</gadget>
<output>6</output>

<gadget id="calculator">6 + 6</gadget>
<output>12</output>

<result>12</result>