In order to encourage residents to save water, a certain city adopts the method of billing in sections, and calculates the water fee of each household on a monthly basis. When the monthly water consumption does not exceed 15 cubic meters, it is charged at 2.8 yuan per cubic meter; 15 cubic meters of which are still charged at 2.8 yuan per cubic meter, and the excess part is charged at 3.6 yuan per cubic meter. The average water fee of Xiaohua's family last month was 3.3 yuan per cubic meter, so how many cubic meters did Xiaohua's family use last month?
某市为了鼓励居民节约用水，采用分段计费的方法，按月计算每户家庭的水费，月用水量不超过15立方米时，按每立方米2.8元计费；月用水量超过15立方米时，其中的15立方米仍按每立方米2.8元收费，超过部分按每立方米3.6元计费．小华家上月的平均水费为每立方米3.3元，那么小华家上月用水多少立方米？
<gadget id="calculator">15 * 3.6</gadget>
<output>54</output>

<gadget id="calculator">2.8 * 15</gadget>
<output>42</output>

<gadget id="calculator">54 - 42</gadget>
<output>12</output>

<gadget id="calculator">3.6 - 3.3</gadget>
<output>0.3</output>

<gadget id="calculator">12 / 0.3</gadget>
<output>40</output>

<result>40</result>