The value of x (length of rectangular field) is :

150m 400 m 200 m 320 m

200 m

Let length = x let breadth = y from question $(x-50)(y+50)=xy$ so $xy-50x+50y-2500=xy$ $x-y=50$   ....(1) $(x-10)(y-20)=xy-5300$ $⇒xy-20x-10y+200=xy-5300$ $2x+y=550$   ...(2) eq. (2) + eq. (1) $⇒3x=600⇒x=200m$