The farmer has to cover 3 times of perimeter of his field with rope. Therefore, required length of rope will b thrice of perimeter of field.

Perimeter of field = 2 × (l + b)

= 2 × (240 m + 180 m)

= 2 × 420 m = 840 m

∴ Total length of rope required = 3 × 840 m = 2520 m