In order to encourage residents to save water, the water supply company stipulates the calculation method of water fee as follows: if the monthly water consumption does not exceed 10 tons, it will be charged at 3.2 yuan per ton; for the part exceeding 10 tons, it will be charged at 5 yuan per ton. Xiaohong's family paid an average of 4 yuan per ton of water last month. How many tons of water did her family use last month?
自来水公司为鼓励居民节约用水，规定如下水费计算方法：每月用水不超过10吨，按每吨3.2元收费；超过10吨的部分按每吨5元计算．小红家上月平均每吨水费交费4元，她家上月用了多少吨水．
<gadget id="calculator">5 * 10</gadget>
<output>50</output>

<gadget id="calculator">10 * 3.2</gadget>
<output>32</output>

<gadget id="calculator">50 - 32</gadget>
<output>18</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">18 / 1</gadget>
<output>18</output>

<result>18</result>