If the width of a rectangle increases by 4 cm, the area increases by 20 cm2; if the length increases by 5 cm, the area increases by 15 cm2. What is the area of the original rectangle?
一个长方形，如果宽增加4厘米，则面积增加20平方厘米；如果长增加5厘米，则面积增加15平方厘米．原来长方形的面积是多少平方厘米？
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>