A hall is 20 m long and 15 m broad. It is surrounded by a verandah of uniform width 2.5 m. Find the cost of flooring only the verandah at Rs 3.50 per square meter.

Introduction / Context:
Rings (frames) around rectangles are handled by subtracting inner area from outer area. The outer dimensions add the uniform border width on both sides of each dimension. The cost equals verandah area times the given rate per square meter.

Given Data / Assumptions:

• Inner (hall) = 20 m × 15 m
• Border width w = 2.5 m (uniform)
• Rate = Rs 3.50 per m^2

Concept / Approach:
Outer rectangle: (20 + 2w) × (15 + 2w) = (20 + 5) × (15 + 5) = 25 × 20. Verandah area = outer − inner. Multiply by cost rate for total cost.

Step-by-Step Solution:
Outer area = 25 × 20 = 500 m^2Inner area = 20 × 15 = 300 m^2Verandah area = 500 − 300 = 200 m^2Cost = 200 × 3.50 = Rs 700

Verification / Alternative check:
Unit check: meters squared times rupees per meter squared gives rupees; numbers are clean, confirming plausibility.

Why Other Options Are Wrong:
Rs 500, 600, 800 correspond to verandah areas 142.86, 171.43, 228.57 m^2 at this rate, which do not match the true area of 200 m^2.

Common Pitfalls:
Adding width once instead of twice per dimension, or mistakenly flooring the hall plus verandah instead of only the verandah area.

Final Answer:
Rs. 700