A city charges monthly natural gas fees according to the following regulations: each household uses 2 yuan per cubic meter of natural gas within 20 cubic meters, and 3.2 yuan per cubic meter for the part exceeding 20 cubic meters. Wang Fang's family paid a total of 62.4 yuan for natural gas last month. How many cubic meters of natural gas did her family use last month?
某城市按以下规定收取每月天然气费：每户用天然气在20立方米以内的部分按每立方米2元收费，超出20立方米的部分按每立方米3.2元收费．王芳家上月共交天然气费62.4元，她家上月用天然气多少立方米？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">62.4 - 40</gadget>
<output>22.4</output>

<gadget id="calculator">22.4 / 3.2</gadget>
<output>7</output>

<gadget id="calculator">7 + 20</gadget>
<output>27</output>

<result>27</result>