In order to save water, a city stipulates that if the monthly water consumption of users does not exceed 20 tons, the water fee will be 0.90 yuan per ton; if it exceeds 20 tons, the excess part will be charged 50% more per ton. Yuan, how many tons does Xiao Ming's family use in May?
为节约用水某市规定：凡每月用户用水量不超过20吨的，每吨收水费0.90元；超过20吨的，超过部分每吨多收50%，如果小明家五月交纳水费23.4元，则小明家五月份用水多少吨？
<gadget id="calculator">20 * 0.9</gadget>
<output>18</output>

<gadget id="calculator">23.4 - 18</gadget>
<output>5.4</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">0.9 * (3/2)</gadget>
<output>1.35</output>

<gadget id="calculator">5.4 / 1.35</gadget>
<output>4</output>

<gadget id="calculator">20 + 4</gadget>
<output>24</output>

<result>24</result>