A class organized three outings in one year, 70% of the students participated in the first outing, 80% of the students participated in the second outing, 90% of the students participated in the third outing, and 12 people participated in the outing. Participated in all three outings. Everyone in the class participated at least twice. How many students are there in this class?
一个班在一年中组织了三次郊游活动，有70%的学生参加了第一次郊游，80%的同学参加了第二次郊游，90%的同学参加了第三次郊游，有12人这三次郊游都参加了，全班每人至少参加两次，这个班有多少学生．
<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(7/10) + (4/5) + (9/10)</gadget>
<output>12/5 = around 2.4</output>

<gadget id="calculator">1 - (12/5)</gadget>
<output>-7/5 = around -1.4</output>

<gadget id="calculator">(4/5) - 1</gadget>
<output>-1/5 = around -0.2</output>

<gadget id="calculator">(7/10) + (-1/5)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(9/10) - 1</gadget>
<output>-1/10 = around -0.1</output>

<gadget id="calculator">(4/5) + (-1/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(7/10) + (-1/10)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(1/2) + (7/10) + (3/5)</gadget>
<output>9/5 = around 1.8</output>

<gadget id="calculator">(-7/5) + (9/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">12 / (2/5)</gadget>
<output>30</output>

<result>30</result>