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 = 800 m

Therefore 8 × breadth = 800

Or, breadth = 800 ÷ 8 = 100 m

So, length = 3 × 100 m = 300 m