At the hearing to adjust the water fee, a representative proposed the following plan: "If the water is less than 5 tons (including 5 tons) per month, it will be charged at 1.2 yuan per ton; if the water is more than 5 tons, but not exceeding 15 tons (including 15 tons) will be charged at 3 yuan per ton; for water exceeding 15 tons (excluding 15 tons), it will be charged at 5 yuan per ton.” Xiao Ming calculated his family’s water bill last month according to this plan, and it happened that the average The water fee is 2 yuan per ton. Please do the math, how many tons did Xiao Ming's family use last month?
在调整水费的听证会上，一位代表提出以下方案："每月用水在5吨以下(包含5吨)，按每吨1.2元收费；用水超出5吨，但不超出15吨(包含15吨)的部分，按每吨3元收费；用水超出15吨(不含15吨)的部分，按每吨5元收费．"小明按此方案计算了一下他家上月水费，正好平均每吨交水费2元．请你算一算，小明家上月用水多少吨？
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">5 * 1.2</gadget>
<output>6</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">3 - 2</gadget>
<output>1</output>

<gadget id="calculator">9 / 1</gadget>
<output>9</output>

<result>9</result>