After the length of a rectangle is reduced by 5 decimeters, the area is reduced by 40 square decimeters, and the remaining part is exactly a square. Find the area of the original rectangle.
一个长方形的长减少5分米后，面积减少40平方分米，且剩下的部分正好是一个正方形，求原长方形的面积是多少．
<gadget id="calculator">40 / 5</gadget>
<output>8</output>

<gadget id="calculator">8 + 5</gadget>
<output>13</output>

<gadget id="calculator">13 * 8</gadget>
<output>104</output>

<result>104</result>