Each issue of a magazine is priced at 5 yuan, and a total of 12 issues are published throughout the year. Some students in a certain class subscribe for half a year, and other students book for a whole year. The total subscription fee is 900 yuan; Q: How many students are there in this class?
某杂志每期定价5元，全年共出12期．某班一些学生订半年，其余学生订全年，共需订费900元；如果订半年的改订全年，而订全年的改订半年，那么共需订费990元．问：这个班共有多少名学生？
<gadget id="calculator">900 - 540</gadget>
<output>360</output>

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">360 / 30</gadget>
<output>12</output>

<gadget id="calculator">9 + 12</gadget>
<output>21</output>

<result>21</result>