It is given in the question that,

Diameter of circular garden = 21 m

∴ Radius of circular garden = Diameter/2 = 21/2 m

Now, the length of rope required for fencing the garden will be equal to the two times the circumference of the garden, because the fencing is done in 2 rounds.

And, One round of circle = the circumference of the circle = 2πr 

= 2 × (22/7) × (21/2)

= 22 × 3

= 66 m

Now, the gardener takes 2 rounds of fence.

⇒ Total length required = 2 × 66 = 132 m

Also, it is given that:

Cost of 1 m of rope = Rs 4

⇒ Cost of 132 m of rope = 4 × 132= Rs 528

∴ the total cost of the rope will be Rs 528