The length, breadth and height of a room are in ratio 3:2:1. If breadth and height are halved while the length is doubled, then the total area of the four walls of the room will

Let length, breadth and height of the room be 3, 2, 1 unit respectively.
Area of walls = 2(l + b) × h = 2(3 + 2) × 1 = 10 sq. unit.
Now, length, breadth and height of room will become 6, 1 and $$\frac{1}{2}$$ respectively.
Now, area of walls = $$2\left( {6 + 1} \right) \times \frac{1}{2}$$   = 7 sq. unit.
% decrease in the area of walls = $$\left( {10 - 7} \right) \times \frac{{100}}{{10}}$$   = 30%