Perimeter of a rectangle = 2 (length + breadth)
Length of the rectangular field = 3 × breadth
Therefore perimeter of field = 2 (3 × breadth + breadth)
= 2 ( 4 × breadth)
= 8 × breadth
The given perimeter = 800m
Therefore 8 × breadth = 800
Or, breadth = 800 ÷ 8 = 100m
So, length = 3 × 100m = 300m