Let breadth be X
so length will be 3x
perimeter=800m
perimeter=2×(l+b)
800=2(X+3x)
800=2×(4x)
800=8x
X=800÷8=100

thus breadth =100 m
and length = 300m................:)

Problem:
The length of a rectangular field is thrice the breadth. The perimeter of the field is 800m. Find the length and breadth of the field.

Solution:

1. Understanding the problem:
We are given that the length of the rectangular field is three times the breadth. We need to find the length and breadth of the field. To solve this, we will use the information about the perimeter of the field.

2. Understanding perimeter:
The perimeter of a rectangle is defined as the sum of the lengths of all its sides. In our case, the perimeter is given as 800m.

3. Expressing the problem in terms of variables:
Let's assume the breadth of the rectangular field as 'b' meters.
Given that the length is three times the breadth, we can express the length as '3b' meters.

4. Calculating the perimeter:
The perimeter of a rectangle can be calculated using the formula: perimeter = 2(length + breadth)
Substituting the given values, we get: 800 = 2(3b + b)

5. Simplifying the equation:
Let's simplify the equation to solve for 'b':
800 = 2(4b)
Dividing both sides by 2, we get: 400 = 4b
Simplifying further, we have: 100 = b

6. Finding the length:
We already know that the length is three times the breadth. Substituting the value of 'b' into the equation, we get:
Length = 3b = 3(100) = 300

7. Final answer:
The length of the rectangular field is 300m and the breadth is 100m.