Area to be painted = Total surface area of the cuboid − Area of the floor
=2(lb+bh+lh)−lb (Since the floor won't be painted)
Cost of painting = Cost per m2 × Area in m2
The perimeter of floor of a rectangular hall is 250 m. If its height is 6 m, find the cost of painting its four walls at the rate of Rs. 20 per meter square.