Two crossing roads inside an 80 m × 60 m lawn, each 10 m wide:\nFind the cost of gravelling the roads at Rs 3 per sq m.

Introduction / Context:
Two strips cross at right angles. Sum their areas and subtract the central overlap counted twice.

Given Data / Assumptions:

• Lawn: 80 m by 60 m.
• Road width w = 10 m.
• Rate = Rs 3 per sq m.

Concept / Approach:
Road area = (80×10) + (60×10) − (10×10).

Step-by-Step Solution:

Verification / Alternative check:
Visualize two rectangles crossing; the overlap is a 10×10 square.

Why Other Options Are Wrong:
3600, 3700, 3800 result from miscounting the overlap or one strip.

Common Pitfalls:
Adding both strips without subtracting the overlapped square.

Final Answer:
Rs. 3900.