Xiaomei used chess pieces to arrange a hollow square array, with 30 chess pieces on each side on the outermost side, a total of 5 layers, so how many chess pieces are used in this square array?
小美用棋子摆了一个空心方阵，最外每边摆30颗棋子，一共摆了5层，那么这个方阵一共用了多少颗棋子？
<gadget id="calculator">30 - 5</gadget>
<output>25</output>

<gadget id="calculator">25 * 5 * 4</gadget>
<output>500</output>

<result>500</result>