A square with a side length of 15 decimeters can increase its area by 1 decimeter if the side length is increased by 1 decimeter.
一个边长为15分米的正方形，边长增加1分米后，面积可增加多少平方分米．
<gadget id="calculator">15 + 1</gadget>
<output>16</output>

<gadget id="calculator">16 * 16</gadget>
<output>256</output>

<gadget id="calculator">15 * 15</gadget>
<output>225</output>

<gadget id="calculator">256 - 225</gadget>
<output>31</output>

<result>31</result>