For a rectangle, if the width is changed to 40 cm and the length remains the same, then its area is reduced by 100 square centimeters; if the width is changed to 50 cm, the length remains the same. Then its area increases by 400 square centimeters. What is the area of the original rectangle?
一个长方形，如果把宽改为40厘米，长不变，那么它的面积就减少100平方厘米；如果把宽改为50厘米，长不变．那么它的面积就增加400平方厘米．原来长方形的面积是多少平方厘米？
<result>210</result>