The school library is full of people. At the beginning, one person has a seat. After leaving (1/3), another 21 people came. There are not enough seats. There are 12 people sharing one seat, which is exactly full. How many seats are there in the library?
学校图书室座满了人，一开始一人一个座位，走了(1/3)，又来了21人，座位不够，有12人合坐一个座位，正好坐满，图书室有几个座位？
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">21 - 6</gadget>
<output>15</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">15 / (1/3)</gadget>
<output>45</output>

<result>45</result>