Cost of fencing per metre = Rs 30
Total cost of fencing = Rs 12000
So, the length of fencing (perimeter) = Total cost/ Cost per metre
= 1200/30
= 400 m
Now, length of fencing = Perimeter of the square field
= 4 × side of the field
Therefore, 4 × side of the field = 400 m
or, side of the field = 400/4 m = 100 m
So, area of the field = 100 m × 100 m = 10000 sq m.