(a) Let the length, breadth and height be l , b and h , respectively. Cost of plastering the four walls = ₹ 48 ⇒ 2(l + b) h = 48 Similarly, Cost of plastering the four walls of new room = 2(2l+ 2b) (2h) =8 (l+ b) h = 4 x 2 (l + b)h = 4 x 48 [∵ 2( l + b) h = 48] = ₹ 192