Final answer:
Without the cost to calculate the plastering, the problem can be understood as a volume and surface area calculation. The cube root of 90 cm³ provides the height of the room, which is also the side length of the square room. The area for plastering is computed based on this dimension, but without the cost value, the question cannot be fully answered.
Step-by-step explanation:
The question posed appears to have some missing information; specifically, we are not provided with the amount to calculate the cost of plastering, which is essential to answer the question directly. However, assuming this is a mathematical problem involving volume and surface area calculations, we can attempt to find the height of the room given its volume. If a room is square, the length, width, and height would be equal. So, if the room contains 90 cm³ of air, we can find the height by taking the cube root of the volume, which is the side length of the cube-shaped room. To find the cost of plastering the four walls, we would need to know the height of the room first, as the area of the walls would depend on this height. The formula for the total surface area of the four walls of a square room is 4 × (height × side length), and then, to find the cost, we would multiply this area by the provided rate of Rs. 30 per m².