Painting walls — Painting the four walls of a room costs Rs. 350. If a new room has each dimension (L, B, H) tripled, what will be the new cost?

Introduction / Context:
Cost to paint four walls is proportional to their total area: A = perimeter * height = 2(L + B) * H. If all dimensions triple, area scales by 3 (perimeter) * 3 (height) = 9.

Given Data / Assumptions:

• Original cost C = Rs. 350
• L → 3L, B → 3B, H → 3H

Concept / Approach:
Area and cost scale by the same factor for the same paint rate.

Step-by-Step Solution:
Scale factor = 3 * 3 = 9New cost = 9 * 350 = Rs. 3150

Verification / Alternative check:
Perimeter scales ∝ (L + B) by 3; height by 3; product is 9.

Why Other Options Are Wrong:
1050 is only 3×; 1400 and 2800 are partial scalings; 4200 is 12×, not 9×.

Common Pitfalls:
Assuming surface area of all six faces; the problem specifies four walls (no ceiling/floor).

Final Answer:
Rs. 3150