During the tree-planting activity of "Green Campus", one day, 15 students came to the school first, and the class leader announced: "We each plant exactly 7 trees." At this time, some students came, and the class leader immediately reassigned tasks Said: "Now we only need to plant 5 trees per person on average to complete the task." Actual reference. How many students add trees?
在创建"绿荫校园"植树活动中，某日，有15名同学先到学校，班长宣布："我们每人正好植7棵树"，这时就又来了一些同学，班长马上重新分配任务说："现在我们平均每人只需要植5棵树就完成任务了"．实际参．加植树的同学有几名？
<gadget id="calculator">7 / 5</gadget>
<output>7/5 = around 1.4</output>

<gadget id="calculator">15 * (7/5)</gadget>
<output>21</output>

<result>21</result>