Correct Answer: INR 6,516

Solution : The total cost of painting its four walls at the rate of INR 25/m$^2$ is INR 3,600.
Rate  = INR 90.50 per m$^2$
The area of four walls, A = $2(l + b)h$, where $l,b$ and $h$ are the length, breadth and height respectively.
Area of floor = $l × b$
Let the length of the room be $6x$.
Breadth of the room $=\frac{6x}{2}= 3x$
Height of the room $=\frac{6x}{3}= 2x$
Cost of painting INR 25/m$^2$
Area of four wall = $(\frac{1}{25}) × 3600$ = 144 m$^2$
So, $2(6x + 3x) × 2x = 144$
⇒ $2x × 18x = 144$
⇒ $36x^2 = 144$
⇒ $x^2 = \frac{144}{36} = 4$
⇒ $x = \sqrt{4} = 2$
Area of floor $=l × b = 6x × 3x = 6 × 2 × 3 × 2 = 72$ m$^2$
Cost of laying carpet on 1 m$^2$ = 90.50
Cost of laying carpet on = 72 m$^2$ = 90.50 × 72 = INR 6,516
Hence, the correct answer is INR 6,516.