There is a square pond with one tree planted at each corner. If 7 trees are planted on each side, how many trees are planted on all four sides?
有一个正方形的池塘，四个角上都栽一棵树，如果每边栽7棵树，四边一共栽多少棵树．
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">28 - 4</gadget>
<output>24</output>

<result>24</result>