A city charges monthly water fees according to the following regulations: if the water used by each household does not exceed 20 cubic meters per month, the water price will be charged at 1.2 yuan per cubic meter; if it exceeds 20 cubic meters, the excess part will be charged at 2 yuan per cubic meter. If the average water price paid by a resident in a certain month is 1.5 yuan per cubic meter, how much cubic meter of water did he use this month?
某市按以下规定收取每月水费：若每月每户用水不超过20立方米，则每立方米水价按1.2元收费；若超过20立方米，则超过部分每立方米按2元收费．如果某户居民在某月所交水费的平均水价为每立方米1.5元，那么他这个月共用了多少立方米的水？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">20 * 1.2</gadget>
<output>24</output>

<gadget id="calculator">40 - 24</gadget>
<output>16</output>

<gadget id="calculator">2 - 1.5</gadget>
<output>0.5</output>

<gadget id="calculator">16 / 0.5</gadget>
<output>32</output>

<result>32</result>