The length of a room floor exceeds its breadth by 20 m. The area of the floor remains unaltered when the length is decreased by 10 m but the breadth is increased by 5 m. The area of the floor (in square metres) is

(c): Let the breadth of floor be x metre. ∴Length =(x+20) metre ∴Area of the floor (x+20)x sq. metre According to question, (x+10)(x+5)=x(x+20) ⇒x2+15x+50=x2+20x ⇒20x=15x+50 ⇒5x=50 ⇒x=10 metre ∴Length =x+20=10+20=30 metre ∴Area of the floor =30×10=300 sq. metre