The side of a square land is (a) = 300 meter
Perimeter (4a) = 300 x 4 = 1200 meter
So the required length of the wall to surround the land = 1200 meter
The area of the land required to build the wall is = 1200x0.3 = 360 square meter
The cost of the building the wall = 360x5000/100 = Rs 18000