On a square wooden board with a circumference of 40 decimeters, saw the largest circle to make a table top. What is the area of this circular table top in square decimeters?
在一块周长为40分米的正方形木板上，锯下一个最大的圆做桌面，这个圆桌面的面积是多少平方分米？
<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

<result>78.5</result>