In the "Young Nobel" defense activity in the school's home laboratory, the fifth grade participated in three groups, the ratio of the first group to the second group was 5:4, and the ratio of the second group to the third group was 3:2 , and the first group is 30 less than the sum of the second and third groups, how many people in the fifth grade participated in the defense activities?
在学校家庭实验室"少年诺贝尔"答辩活动中，五年级分三组参加，第一组与第二组人数的比是5：4，第二组和第三组人数的比是3：2，且第一小组比第二、三组人数总和少30人，五年级参加答辩活动一共有多少人．
<gadget id="calculator">12 + 8</gadget>
<output>20</output>

<gadget id="calculator">15 + 12 + 8</gadget>
<output>35</output>

<gadget id="calculator">20 / 35</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">15 / 35</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(4/7) - (3/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">30 / (1/7)</gadget>
<output>210</output>

<result>210</result>