The sixth grade students of Huangshan Primary School were divided into three groups to participate in the tree planting. The ratio of the number of people in the first group to the second group is 5:4, and the ratio of the number of people in the second group to the third group is 3:2. It is known that the number of people in the first group is 15 less than the sum of the numbers in the second and third groups. How many people in the sixth grade participated in the tree planting?
黄山小学六年级的同学分三组参加植树．第一组与第二组的人数的比是5：4，第二组与第三组人数的比是3：2．已知第一组的人数比二、三组人数的总和少15人．六年级参加植树的共有多少人．
<gadget id="calculator">8 - 15</gadget>
<output>-7</output>

<gadget id="calculator">12 + (-7)</gadget>
<output>5</output>

<gadget id="calculator">15 / 5</gadget>
<output>3</output>

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

<gadget id="calculator">3 * 35</gadget>
<output>105</output>

<result>105</result>