In order to allow residents to stagger the peak consumption of electricity and ensure industrial electricity consumption, the municipal power supply company introduced a time-of-use electricity price. During the day, from 8:00 am to 9:00 pm, residents are charged according to the "valley" price, and the price of electricity per kWh is 0.54 yuan. From 9:00 p.m. to 8:00 a.m. the next day, electricity will be charged at a "flat" price, with an electricity price of 0.45 yuan per kWh. Xiaoming's family shared 80 kWh of electricity in May, and paid 40.5 yuan for electricity. How many kWh of electricity did his family use at the "valley" price this month?
市供电公司为了让居民错开用电高峰，保证工业用电，特推出分时电价．一天中，早上8点--晚上9点居民用电按"谷"价收费，每千瓦时电价0.54元．晚上9点--次日8点用电按"平"价收费，每千瓦时电价0.45元．小明家五月份共用电80千瓦时，交电费40.5元，他家这个月中"谷"价用电多少千瓦时．
<gadget id="calculator">80 * 0.54</gadget>
<output>43.2</output>

<gadget id="calculator">43.2 - 40.5</gadget>
<output>2.7</output>

<gadget id="calculator">0.54 - 0.45</gadget>
<output>0.09</output>

<gadget id="calculator">2.7 / 0.09</gadget>
<output>30</output>

<gadget id="calculator">80 - 30</gadget>
<output>50</output>

<result>50</result>