A total of 30 teachers and students went to participate in the voluntary tree planting activity. The teacher planted 5 trees each, and the students planted 3 trees each, exactly 100 trees were planted in total. How many teachers participated in tree planting?
老师和学生一共30人去参加义务植树活动．老师每人植5棵，学生每人植3棵，正好一共植了100棵．参加植树的老师有多少人．
<gadget id="calculator">100 - 90</gadget>
<output>10</output>

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<result>5</result>