In a rectangular field, the ratio of its length to its breadth is 5:4. The breadth is 20 m less than the length. Find the perimeter of the rectangular field in metres, keeping the ratio condition and the 20 m difference condition unchanged.

Introduction / Context:
This question checks your ability to convert a length:breadth ratio into actual dimensions using a given difference, and then compute the perimeter. The typical approach is to represent length and breadth as proportional values (5x and 4x), use the difference to find x, and then compute perimeter using 2 * (l + b).

Given Data / Assumptions:

• Length:breadth = 5:4
• Breadth is 20 m less than length, so l - b = 20
• Perimeter of rectangle = 2 * (l + b)

Concept / Approach:
Let l = 5x and b = 4x. Then l - b = x. Use the 20 m difference to solve for x and find l and b, then apply the perimeter formula.

Step-by-Step Solution:

Verification / Alternative check:
Check the difference: 100 - 80 = 20 m, matches the condition. Check the ratio: 100:80 simplifies to 5:4, also correct. Therefore the perimeter computed from these correct dimensions must be correct.

Why Other Options Are Wrong:

Common Pitfalls:
Students sometimes treat 20 m as 20% or add/subtract it from the ratio directly. Another mistake is using area formulas instead of perimeter. Also, some forget to multiply (l + b) by 2 when computing perimeter.

Final Answer:
360 m