The fourth grade students of Yuhong Elementary School participated in the tree planting activities, and they were divided into groups of 15 or 18, and they were just finished. At least how many people in the fourth grade participated in the tree planting.
育红小学四年级同学参加植树活动，按15人分一组或18人分一组都正好分完，四年级参加植树的至少有多少人．
<gadget id="calculator">2 * 3 * 3 * 5</gadget>
<output>90</output>

<result>90</result>