A rectangular lawn is 120 meters long and 90 meters wide. Trees are planted around it. It is required to plant trees at the four corners and the midpoints of each side, and the distance between two adjacent trees is equal. May I ask: at least How many trees will be planted?
一块长方形草地，长120米，宽90米，现在在它的四周种树，要求四个角和各边中点都要求种树，且相邻两棵树之间的距离都相等，请问：最少要种多少棵树？
<gadget id="calculator">120 + 90</gadget>
<output>210</output>

<gadget id="calculator">210 * 2</gadget>
<output>420</output>

<gadget id="calculator">420 / 15</gadget>
<output>28</output>

<result>28</result>